Wireless energy transfer system for an implantable medical device using a midfield coupler

ABSTRACT

Described herein are devices, systems, and methods for wireless power transfer utilizing a midfield source and implant. In one variation, a midfield source may be realized by a patterned metal plate composed of one of more subwavelength structures. These midfield sources may manipulate evanescent fields outside a material (e.g., tissue) to excite and control propagating fields inside the material (e.g., tissue) and thereby generate spatially confined and adaptive energy transport in the material (e.g., tissue). The energy may be received by an implanted device, which may be configured for one or more functions such as stimulation, sensing, or drug delivery.

RELATED APPLICATION

This application is a continuation of Ser. No. 15/200,543, filed Jul. 1,2016, and titled “MIDFIELD COUPLER,” which application is a continuationunder 35 U.S.C. §1.111 (a) of PCT Application Serial NumberPCT/US2015/030995, filed May 15, 2015, and titled “MIDFIELD COUPLER”,which are incorporated herein by reference in their entirety.

FIELD

This disclosure is generally related to wireless power transfer. Morespecifically, described herein are devices, systems, and methods formidfield coupling to an implanted device, such as a microstimulator,sensor, ablation, or drug delivery device.

BACKGROUND

Although considerable progress has been made in energy storagetechnologies, batteries remain a major obstacle to miniaturization ofimplantable electronics. As a result, current implantable electricalstimulation systems typically include a large impulse generatorcontaining a titanium case enclosing the battery and circuitry used togenerate the electrical pulses. The impulse generator is typicallyimplanted within a cavity in the body such as under the clavicle, belowthe rib cage, in the lower abdominal region, or in the upper buttock.Electrical pulses are then delivered to a targeted nerve or muscleregion via leads routed underneath the skin or through a blood vessel.Problems associated with this current approach include pocketinfections, lead dislodgment, lead fracture or perforation, muscle teardue to implanting in or pulling out the leads, and limited locations forthe placement of the electrodes. In addition, the lifetime of thesedevices is burdensomely limited, requiring periodic surgical replacementonce the battery unit is depleted.

Alternatively, energy can be wirelessly transferred from an externalsource, but the ability to transfer power to small implanted devicesand/or devices located beyond superficial depths remains challenging.Most of the known wireless powering methods for implantable electronicsare based on the near-field coupling method, and these and othersuggested methods suffer from a number of disadvantages. The powerharvesting structure in the implanted device (e.g., the coil(s) orantenna(s)) is typically large. The largest dimension is typically onthe order of a centimeter or larger. The coils external to the body innear-field coupling methods are also typically bulky and inflexible.This presents some difficulties with regard to the incorporation of theexternal device into daily life. The intrinsic exponential decay of thenear field limits miniaturization of the implanted device beyondsuperficial depths (greater than 1 cm). On the other hand, the radiativenature of the far field severely limits the energy transfer efficiency.It may therefore be desirable to have devices and methods fortransmitting wireless power to small implantable devices, andcorresponding small implantable devices suitable for less invasivedelivery methods.

BRIEF SUMMARY

Described herein are devices, systems, and methods for wireless powertransfer utilizing a midfield source and implant. In one variation, amidfield source (coupler) may be realized by a patterned metal platecomposed of one of more subwavelength structures. These midfield sourcesmay manipulate evanescent fields outside a material (e.g., tissue) toexcite and control propagating fields inside the material and therebygenerate spatially confined and adaptive energy transport in thematerial. The energy may be received by an implanted device, which maybe configured for one or more functions such as stimulation, ablation,sensing, or drug delivery, among other functions.

In one variation, the devices described herein are midfield sources. Themidfield sources may comprise a midfield plate and one or moreexcitation ports. The midfield plate may comprise a planar surface andone or more subwavelength structures. The midfield source may beconfigured for wireless power transmission through tissue. In somevariations, the midfield source comprises a planar structure comprisinga metal, at least one of a slot or metal strip, and an excitation portcoupled to the slot or metal strip, wherein the source is capable ofgenerating an electromagnetic field with a spatial frequency spectrum ofthe field adjacent to the source having non-negligible components thatlie in the range of k₀≦√{square root over (k_(x) ²+k_(y) ²<k_(muscle))}.In some of these variations, the device comprises at least two slots orat least two metal strips. In some of these variations, the two slots ortwo metal strips are excited by the same excitation port. In some ofthese variations the two slots or two metal strips are excited by thesame excitation port using a microstrip transmission line. In somevariations, the device further comprises a controller for dynamicallyshifting a focal region of the electromagnetic field. In somevariations, the device comprises eight slots, wherein the eight slotsare arranged in pairs of an intersecting linear slot and curved slot. Insome of these variations, each pair of slots is excited by a singleexcitation port.

Also described herein are systems for wirelessly powering an implantthrough tissue. In some variations, the systems may comprise a midfieldsource comprising a midfield plate comprising a planar structure and asubwavelength structure, and an excitation port for exciting thesubwavelength structure, and an implant comprising a receiver coil,wherein the midfield source is configured to transmit power to theimplant through propagating modes of the tissue. In some of thesevariations, the source is capable of generating an electromagnetic fieldwith a spatial frequency spectrum of the field adjacent to the sourcehaving non-negligible components that lie in the range of k₀≦√{squareroot over (k_(x) ²+k_(y) ²<k_(muscle))}. In some of these variations,the midfield plate comprises a flexible substrate. In some of thesevariations, the flexible substrate comprises an adhesive and isconfigured to be attached to a patient's skin. In some of thesevariations, the diameter of the implant is less than 3 mm. In some ofthese variations, the implant comprises an electrode. In somevariations, the implant comprises a sensor. In some variations, themidfield source comprises a controller for dynamically shifting a focalregion of the electromagnetic field in response to feedback from thesensor.

Also described herein are methods for wireless transmitting power to animplant through a material. In some variations, the method comprisesgenerating an electromagnetic field with a source, and wirelesslytransferring energy to a receiver coil of the implant through thematerial, wherein the spatial frequency spectrum of the field adjacentto the source comprises non-negligible components that lie in the rangeof k₀≦√{square root over (k_(x) ²+k_(y) ²<k_(muscle))}. In somevariations, the source and the implant are at least 5 cm apart, and theimplant has a diameter of less than 3 mm. In some of these variations,the power transfer to the coil is at least 10 μW when 500 mW are coupledinto the material. In some variations, the method further compriseswirelessly transferring energy to a second receiver coil of a secondimplant. In some variations, the method further comprises adjusting afocal region of the electromagnetic field.

Also described herein are wireless power systems comprising a sourcecomprising one or more subwavelength structures configured to wirelesslytransmit power by manipulating evanescent fields outside of tissue togenerate a spatially focused field in the tissue, an implant configuredto receive the wireless power from the external module, the implantcomprising at least one sensor or stimulator. In some variations, thesensor is selected from the group consisting of a thermal sensor, achemical sensor, a pressure sensor, an oxygen sensor, a PH sensor, aflow sensor, an electrical sensor, a strain sensor, a magnetic sensor,and an imaging sensor. In some variations, the stimulator is selectedfrom the group consisting of an electrical stimulator, an opticalstimulator, a chemical stimulator, and a mechanical stimulator. In somevariations, the implantable device comprises a modular design thatallows interchangeable sensors and/or stimulators. In some variations,the one or more subwavelength structures are selected from the groupconsisting of a patch, a PIFA, a slot, a cross slot, an aperture coupledcircular slot, and a half slot. In some variations, the source isconfigured to adjust a position of a focal point of the spatiallyfocused field. In some of these variations, the implant comprises asensor to detect a power level of received wireless energy, andcomprises a transmitter to provide feedback to the external module toautomatically adjust the position of the focal point to optimizewireless power transmission. In some variations, the implant isconfigured to be implanted on, in, or near a heart to apply leadlesspacing to the heart. In some variations, the implant is configured to beimplanted on, in, or near a brain to apply deep brain stimulation to thebrain. In some variations, the implant is configured to be implanted on,in, or near a spinal cord to apply stimulation to the spinal cord. Insome variations, the implant is configured to be implanted on, in, ornear a muscular tissue of the tongue to apply stimulation to the tongueto treat obstructive sleep apnea.

Also described herein is a method of cardiac pacing, comprisingimplanting a wireless power receiving module in, on, or near a heart,transmitting a midfield propagating wave to the wireless power receivingmodule to power the module, sensing a parameter of the heart with themodule; and providing electrical pacing to the heart with the module.

Also described herein is a method of deep brain stimulation, comprisingimplanting a wireless power receiving module in, on, or near a brain,transmitting a midfield propagating wave to the wireless power receivingmodule to power the module, sensing a parameter of the brain with themodule, and providing stimulation to the brain with the module.

Also described herein is a method of stimulating tissue, comprising:implanting a wireless power receiving module into tissue, transmitting amidfield propagating wave to the wireless power receiving module topower the module, and sensing a parameter of the tissue with the module;and providing stimulation to the tissue with the module. In somevariations, the method further comprises adjusting a focal point of thepropagating wave to optimize wireless power transmission to the module.In some variations, the transmitting step comprises transmitting thewave with a subwavelength structure that produces a magnetic fieldperpendicular to the wave and parallel to a tissue interface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic side view of a source located above aninterface between air and a high-index material.

FIG. 2A shows schematic for power transfer to a coil mounted on thesurface of the heart.

FIGS. 2B and 2C show a magnetic field generated by a source currentdensity optimized for power transfer across a multilayered material.

FIGS. 3A and 3C show a magnetic field generated by a midfield source forpower transfer across the multilayered material of FIG. 2B. FIGS. 3B and3D show a magnetic field generated by a near-field source for powertransfer across the multilayered material of FIG. 2B.

FIGS. 4A and 4B show the magnetic field generated by a midfield sourceand a near-field source, respectively, in the absence of themultilayered material of FIG. 2B.

FIG. 5 plots performance curves for midfield and near-field coupling fora fixed power transfer efficiency.

FIGS. 6A-6C show perspective top views of variations of midfield sourcesdescribed here placed near representative tissue.

FIGS. 7A-7B show schematics of subwavelength structures described here.

FIGS. 8A-8F show schematics of subwavelength structures described here.

FIGS. 9A-9B show schematics of subwavelength structures described here.

FIGS. 10A-10B show schematics of midfield plates described here.

FIGS. 11A-11E show schematics of subwavelength structures describedhere.

FIGS. 12A-12B show architectures for multiple excitation ports.

FIGS. 13A-13B show schematics of a midfield source comprising foursub-wavelength structures fed by one excitation port.

FIG. 14 shows a schematic of a midfield source comprising an array ofsub-wavelength structures that approximates an optimal source.

FIG. 15 shows a schematic of a midfield source comprising an array ofsub-wavelength structures that approximates an optimal source.

FIGS. 16A-16C show the performance of the midfield source of FIG. 15.FIG. 16A shows the spatial frequency spectrum of the output field. FIG.16B shows the power transfer efficiency. FIG. 16C shows the scatteringspectrum.

FIG. 17 show the spatial frequency spectra of the output fields from thesub-wavelength structures of FIG. 7A.

FIGS. 18A-18E show the magnetic fields produced by the subwavelengthstructures in FIGS. 11A, 11B, 11D, 11C, and 11E, respectively.

FIG. 19 shows the magnetic field generated by a conventional inductivelycoupled loop source.

FIG. 20 shows field patterns with spatially shifted focal points createdby adjusting the relative amplitude and phases between the port signalsin the midfield source of FIG. 15.

FIGS. 21A-21B show an architecture for a controller of a midfieldsource.

FIGS. 22A-22E show the effect of real-time dynamic focusing.

FIG. 23A-23C show photographs of an example of an implant as describedhere, on a human finger, before epoxy encapsulation next to a cathetersheath for size comparison, and inserted in the lower epicardium of arabbit, respectively. FIG. 23D shows the operating frequency of amidfield source applied to the implant of FIG. 23C. FIG. 23E shows anECG of the rabbit of FIG. 23C. FIG. 23F shows the autocorrelationfunction of both the off-resonance and resonant sections of the ECGsignal of FIG. 23E.

FIGS. 24A-24C show circuit schematic for a power receiver, a datatransceiver, and a stimulator and sensor, respectively.

FIGS. 25A-25C show circuit schematics for an implant configured tostimulate tissue.

FIGS. 26A-26B show magnetic resonance imaging reconstructions of implantpositions within porcine tissue volumes.

FIGS. 27A-27B shows a specific absorption rate measurement setup. FIGS.27C-27D show specific absorption rate distribution from the setup ofFIG. 27A.

FIGS. 27C-27D show measurements of the power transfer in the setup ofFIG. 27A and that the power transfer is well below the threshold. FIG.27E shows received power when the power coupled into tissue in the setupof FIG. 27B and FIG. 27F shows the received power is at or below themaximum permitted level of exposure.

FIGS. 28A-28C show midfield sources attached to a patient.

FIG. 29 shows a receiver device implanted in tissue.

FIG. 30 shows a time domain multiplexed communication system.

FIG. 31 shows an amplitude and phase shifting network.

FIG. 32 shows a midfield coupler attached to tissue.

FIG. 33 shows a method for customizing a midfield coupler.

DETAILED DESCRIPTION

Difficulties in achieving wireless power transfer may occur in themismatch between the size of the implantable devices/sensors and thepower transfer source, the depth of the devices/sensors in a patient,and additionally the spatial arrangement of the devices/sensors relativeto the power transfer source. Described here are devices, systems, andmethods for wireless powering of microimplants that may overcome theseproblems. The power sources described herein may generate anon-stationary evanescent field, which may induce energy transferthrough the propagating modes of the tissue volume. The region where theenergy transfer, termed the electromagnetic midfield, is about awavelength's distance from the source, where the wavelength correspondsto that of the biological material.

In conventional wireless powering approaches using near-field coupling(inductive coupling and its resonant enhanced derivatives), theevanescent components outside tissue (near the source) remain evanescentinside tissue, which does not allow for effective depth penetration.Unlike conventional near-field coupling, energy from the midfield sourceis primarily carried in propagating modes and, as a result, thetransport depth is limited by environmental losses rather than theintrinsic exponential decay of the near-field. Theoretical studies haveshown that energy transfer implemented with these characteristics can betwo to three orders of magnitude more efficient than near-field systems.In midfield coupling, the tissue may act as a dielectric to tunnelenergy, and coherent interference of the propagating modes may confinethe field at the focal plane to dimensions much smaller that the vacuumwavelength, with a spot size subject to the diffraction limit in ahigh-index material. By locating an implant at this high energy densityregion, the implant may be able to be made orders of magnitudes smaller,and may be able to be placed deeper within a material, than in systemsusing traditional wireless powering methods. Indeed, due to the highdielectric permittivity exhibited by biological tissue at microwavefrequencies, the power sources described herein may be configured todeliver electromagnetic energy to implantable devices at the scale of amillimeter or less implanted deep inside the body.

Theoretical Basis

The physics underlying the devices, systems, and methods described herearises from an optimization methodology that bounds the performanceachievable by any physical realization of a power source. Power transferoccurs when a source generates fields whose interaction with a receivercoil results in work extraction by a load in the receiver circuit. For asubwavelength receiver coil, only the lowest order mode is important,and the power transfer mechanism can be described by electromagneticinduction characteristic of dynamic magnetic field interactions. Thepower transferred to the coil is given by

$P_{S\; C} = {\int{{\mathbb{d}^{3}r}\;{{M_{C}(t)} \cdot \frac{\mathbb{d}{B_{S}(t)}}{\mathbb{d}t}}}}$where B_(S) is the magnetic field generated by the source and M_(C) theinduced magnetization due to current in the coil. The electric andmagnetic fields generated by a time-harmonic current density J_(S) onsurface of the source conductor can be solved by decomposing the currentdensity into its spatial frequency components, each of which obey theusual laws for refraction and reflection across planar interfaces. Usingphasor notation with a time dependence of exp(−iωt), the efficiency maybe defined as:

$\eta = \frac{{{\int{{\mathbb{d}^{3}r}\;{M_{C}^{*} \cdot B_{S}}}}}^{2}}{\left\lbrack {{\int{{\mathbb{d}^{3}r}\;{Im}}} \in {(\omega){E_{S}}^{2}}} \right\rbrack\left\lbrack {{\int{{\mathbb{d}^{3}r}\;{Im}}} \in {(\omega){E_{C}}^{2}}} \right\rbrack}$Formally, η is the ratio of power available at the coil to the totalabsorbed power. This equation considers only dissipation in tissue:other losses, such as radiation and ohmic loss, may arise in practice,but the amount of power that can be coupled into the body may beessentially limited by electric field-induced heating.

This efficiency is intrinsic to the fields in the tissue multilayerstructure and gives an upper bound on the efficiency that can beobtained. This expression for efficiency can be derived using coupledmode theory: The exchange of energy between the source and receiver isdescribed by the equations{dot over (a)} _(S)(t)=(iω _(S)−Γ_(S))a _(C)(t)+κa _(S)(t){dot over (a)} _(C)(t)=(iω _(C)−Γ_(C)−Γ_(L))a _(C)(t)+κa _(S)(t)

where a_(n) are amplitudes normalized such that |a_(n)|² corresponds tothe energy in the structure, Γ_(n) the intrinsic decay rates, Γ_(L) therate of work extraction by the load on the receiver, and κ the couplingcoefficient. It may be advantageous to operate with the source andreceiver in resonance ω=ω_(S)=ω_(C). The efficiency of power transfer isdefined as

$\eta^{\prime} = \frac{\Gamma_{L}{a_{S}}^{2}}{{\Gamma_{S}{a_{S}}^{2}} + {\left( {\Gamma_{C} + \Gamma_{C}} \right){a_{S}}^{2}} + {{Re}\left( {\kappa\; a_{S}^{*}a_{C}} \right)}}$In the limit of weak coupling |κ|²/Γ_(S)Γ_(C)<<1, the expression reducesto

$\eta^{\prime} = {\frac{{\kappa }^{2}}{\Gamma_{S}\Gamma_{C}}\frac{\Gamma_{C}\Gamma_{L}}{\left( {1 + {\Gamma_{C}/\Gamma_{L}}} \right)^{2}}}$which is the product of two efficiencies. The left hand factor can beunderstood as the efficiency of power transfer to the coil in absence ofthe load. The right-hand factor corresponds to the efficiency of powerextraction by the load—this factor is maximized when theimpedance-matching condition Γ_(C)=Γ_(L) is satisfied. From standardpower arguments, it can be shown that the left-hand efficiency is givenby

$\frac{{\kappa }^{2}}{\Gamma_{S}\Gamma_{C}} = \frac{{{\int{{\mathbb{d}^{3}r}\;{B_{S}^{*} \cdot \; M_{C}}}}}^{2}}{\left\lbrack {{\int{{\mathbb{d}^{3}r}\;{Im}}} \in {(\omega){E_{S}}^{2}}} \right\rbrack\left\lbrack {{\int{{\mathbb{d}^{3}r}\;{Im}}} \in {(\omega){E_{C}}^{2}}} \right\rbrack}$which is the efficiency defined above. Equivalent expressions can beobtained using other formalisms for coupled electrical systems, such asa two-port lumped element network.

Source J_(S) may be chosen to maximize efficiency. The global optimummay be analytically solved for a specified powering configuration bydefining an electric current with components tangential to a planebetween the source structure and tissue. For every source, theelectromagnetic equivalence theorem enables such a two-dimensionalcurrent density to be chosen from the overall set S that isindistinguishable in the lower z<0 half-space from the physical sourceof the fields. Remarkably, solution to the optimization problemmaximize_(j) _(S) _(∈S)η(J_(S)) can be found in closed-form as aconsequence of the vector space structure of S. In contrast with localoptimization algorithms, this solution obtained is a rigorous bound onthe performance that can be achieved by any physical realization of thewireless powering source.

This theory may be applied to systems comprising an external powersource and implanted device, as described herein. FIG. 1 shows aschematic of a side view of a source 102 located above an interfacebetween air 104 and a high-index material 106. The source 102 mayproduce an in-plane source current density J_(S). This source currentmay generate an electric field E_(S) and magnetic field H_(S) asdescribed by the dyadic Green functions, G_(E) and G_(H):E _(S) =iωμ ₀ ∫G _(E)(r−r′)J _(S)(r′)dr′H _(S)(r)=∫G _(B)(r−r′)J _(S)(r′)dr′where ω is the angular frequency and μ₀ is the permeability of air.Applying the Fourier transform in each of the transverse coordinatesyields the spatial frequency spectra of the fields:E _(S)(k _(x) ,k _(y) ,z)=iωμ ₀ G _(E)(k _(x) ,k _(y) ,z)J _(S)(k _(x),k _(y))H _(S)(k _(x) ,k _(y) ,z)=G _(H)(k _(x) ,k _(y) ,z)J _(S)(k _(x) ,k_(y)).If k₀ is the wavenumber of air, the spectral components in air wherek _(x) ² +k _(y) ² >k ₀ ²correspond to the evanescent fields in air.

Energy transfer from the source 102 to a receiver coil located withinthe high-index material 106 may occur when the source 102 generatesfields whose interaction with the coil results in work extraction by aload in the receiver circuit.

The time-averaged power transferred to the coil may be given by<P _(SC)>=½Re[iω∫μ ₀ H* _(S)(r)·M _(C)(r)dr]where M_(C) is the induced magnetization on the coil. The power transferefficiency in terms of the fields is thus defined as

$\eta = \frac{{{\int{\mu_{0}{{H_{S}^{*}(r)} \cdot {M_{C}(r)}}{\mathbb{d}r}}}}^{2}}{\int{{{Im}\left\lbrack {ɛ(r)} \right\rbrack}{{E_{S}(r)}}^{2}{\mathbb{d}r}{\int{{{Im}\left\lbrack {ɛ(r)} \right\rbrack}{{E_{C}(r)}}^{2}{\mathbb{d}r}}}}}$where ∈ is the permittivity of the material and E_(C) is the electricfield generated by M_(C). Formally, η is the ratio of power available atthe coil to the total power dissipated in the material.

The efficiency of power transfer as defined by η above may be changed bythe choice of source J_(S). When the air-material medium is a multilayerstructure, the global optimum may be analytically solved. By exploringsuch global solutions across a range of frequencies with appropriatedispersion models for biological materials, the optimal power transferfor particular biological tissue or tissues may be determined. Forexample, the theory described above may be used to determine the optimalpower transfer for an approximation of the chest wall structure. FIG. 2Aillustrates a schematic for power transfer to a coil mounted on thesurface of the heart. The chest wall may be approximated by a multilayerstructure as shown in FIG. 2B. As shown in FIG. 2A, the poweringconfiguration may consist of a source 202 (described in more detailbelow) positioned above the skin and a receiver coil 204 (described inmore detail below) inserted in the cardiac tissue layer. The optimalpower transfer for this approximated chest wall structure may bedetermined by solving the optimal source J_(S) across a range offrequencies with appropriate dispersion models for the appropriatebiological materials.

In this example, optimal power transfer may occur at 1.6 GHz. Todetermine this, theoretical efficiency versus frequency curves weregenerated by solving for the optimal η in a multilayer model of tissue(1 cm air gap, 4 mm skin, 8 mm fat, 8 mm muscle, 16 mm bone, ∞ heart)across a wide search range (10 MHz to 4 GHz) for coils oriented in the xand z directions. The upper frequency bound was selected to be about theself-resonance frequency of the coil. The coil losses were taken intoaccount using an analytical model for a loop of wire embedded in uniformtissue, as well as impedance matching by imposing the constraint Q<10,where Q is the quality factor. Using the Debye dispersion model for eachtissue type, the peak efficiency was found to occur at 1.6 GHz.

As shown in FIG. 2B, the solution for J_(S) may yield a highlyoscillatory electric current density 206 that may cause the output fieldto converge on the receiver coil 204. The fields were calculated fromthe spectral components of an in-plane source current densityJ_(S)(k_(x),k_(y)) using the dyadic Green's function method. This methodreduced to a simple transfer function because the plane-wave componentsare Eigen functions of propagation in the multilayer structure. At eachdepth z, for example, a dyad G_(H)(k_(x),k_(y),z) was applied tocalculate the magnetic field H(k_(x),k_(y),z)=G_(H)(k_(x),k_(y),z)J_(C)(k_(x),k_(y)). An inverse Fourier transform yields the fields ateach depth.

FIG. 2C plots the spatial frequency spectra at depth planescorresponding to the source (z=0), the skin surface (z=Z_(skin)=−1 cm),and the coil (z=z_(coil)=−5 cm). At the depth plans corresponding to thesource and the skin surface, the output fields 208 and 210,respectively, may be composed of significant evanescent componentscorresponding to√{square root over (k _(x) ² +k _(y) ²)}>k ₀.Near the receiver coil 204, the output field 212 may be composed ofsignificant propagating modes corresponding to√{square root over (k _(x) ² +k _(y) ²)}≦k _(muscle),where k_(muscle) is the wavenumber in muscle tissue. Thus, due to thehigh dielectric permittivity exhibited by biological materials atmicrowave frequencies, complete control of the propagating modes intissue may be achieved when the source “lens” affects evanescent wavecomponents that lie in the range ofk ₀≦√{square root over (k _(x) ² +k _(y) ²)}<k ₀.

FIG. 3A shows a time snapshot of the output magnetic field from amidfield source having such a lens, while FIG. 3C shows the normalizedspatial frequency spectrum of the output field adjacent to the midfieldsource. In contrast, FIG. 3B shows a time snapshot of the output fieldfrom a corresponding near-field source comprising a coil having adiameter of 4 cm and operating at 10 MHz, normalized such that themaximum electric field in the tissue is the same in FIGS. 3A and 3B.FIG. 3D shows the normalized spatial frequency spectrum of the outputfield adjacent to the near-field source. As can be seen in FIG. 3B, thefields from the near-field source decay much more quickly than thefields from the midfield source, and are not propagating. Similarly, thespatial frequency spectrum shown in FIG. 3C shows k₀≦√{square root over(k_(x) ²+k_(y) ²)}<k_(muscle), while most of the components shown inFIG. 3D lie in √{square root over (k_(x) ²+k_(y) ²)}>>k_(muscle)>k₀>k₀which are non-propagating in both air and tissue material.

Unlike conventional near-field coupling, midfield powering exploits thehigh dielectric permittivity exhibited by biological materials atmicrowave frequencies to facilitate the transport of energy. Thus, thebenefit may not be seen in instances in which tissue is not presentbetween the source and the receiver coil. For example. FIG. 4A plots theoutput magnetic field from the midfield source when the tissue materialis removed, while FIG. 4B plots the output magnetic field from thenear-field source when the tissue material is removed. As can be seen,the magnetic fields from the midfield source shown in FIG. 4A may decaymore quickly than when the tissue material is present, and are notpropagating, as was shown in FIG. 3A.

The difference between the power transfer using near-field and midfieldcoupling may also be illustrated by performance curves—diameter of thereceive coil versus power transfer range. FIG. 5 plots the performancecurves for a fixed power transfer efficiency η=10⁻³ and circuit load of10Ω. To achieve this efficiency at a given wavelength, the coil size andoperating depth is constrained to lie under the performance curve. Thecurves may be generated by solving for optimal power transfer in anair-muscle half-space for the indicated depths and wavelength in tissue.For example, for an operating depth (distance between the source and thereceive coil) of 5 cm, a near-field source may require the diameter ofthe receiver coil to be at least 15 mm, while a midfield source may onlyrequire the diameter of the receiver coil to be at least 2 mm. Thisperformance curve indicates that transporting electromagnetic energydeep in the body to implantable devices at the scale of a millimeter orless is possible.

Devices

Described herein are devices, systems, and methods for realizing amidfield source as described theoretically above. The implementation ofa source having the required lensing properties as laid out aboutrequires electromagnetic structures more complex than conventional coilor dipole elements. In one variation, a midfield source may be realizedby midfield plate, which may comprise one of more subwavelengthstructures, and excitation ports configured to excite the subwavelengthstructures. These midfield sources may manipulate evanescent fields nearthe source to excite and control propagating fields inside a material(e.g., tissue) and thereby generate a spatially focused and adaptivesteering field in the material. The energy may be received by animplanted device, described in more detail below.

The systems described herein may allow for wireless power transfer toimplanted devices at depths unattainable with conventional inductivecoupling technology. Moreover, the implants may be able to be muchsmaller (e.g., by one, two, or three orders of magnitude) than theexternal power source, and much smaller (e.g., by one, two, or threeorders of magnitude) than their depth within a material (e.g., tissue).The power that may be transferred to an implant via the systemsdescribed herein may also be sufficient to power the delivery of stimuliand/or complex electronics in the implant.

External Module (Midfield Source)

Described herein are midfield sources configured to generate power thatmay be wirelessly transferred to implants. In some variations, theentire midfield source may be integrated into a hand-held device. Assuch, the midfield source may be suitable for on-demand use. In othervariations, the midfield source may be configured to be worn on the bodyor affixed to the skin surface. FIGS. 6A-6C show perspective top viewsof variations of midfield sources 602 placed near representative tissue604. As shown there, each midfield source 602 may comprise a midfieldplate, which may comprise a planar surface 606 and one or moresubwavelength structures 608. The one or more subwavelength structures608 on the midfield plate may be excited by one or more radio-frequencyports, which together may form the midfield source, as described in moredetail below.

Planar Surface

In some variations, the planar surfaces of the midfield sourcesdescribed herein may comprise a solid substrate or plate. For example,the planar surface may in some variations comprise glass epoxylaminates, such as FR-4, which may comprise feed and patterned copperlayers. In other variations, the planar surface may comprise Rogers orceramic for lower substrate loss. The planar surface may have agenerally planar shape, and may have any suitable dimensions. Thethickness may depend on the number of metal layers in the substrate, andmay range from about 1 mm to about 3 cm. In some variations, the solidsubstrate may be approximately 6 cm by 6 cm, and have a thickness ofabout 1.6 mm.

In other variations, the planar surface may comprise a flexiblesubstrate. In some variations in which the planar surface comprises aflexible substrate, the flexible substrate may be an ultrathin flexiblesubstrate, and may be configured to conform to an irregular or curvedsurface, such as a patient's skin. For example, the planar surface mayin some variations comprise ultrathin FR-4. In some variations, theflexible substrate may have a thickness of about 10 μm to about 1 mm.More specifically, in some variations, the flexible substrate may have athickness of about 100 μm. The thickness may depend on the number ofmetal layers in the substrate and the isolation between differentlayers.

The planar surface may be configured to adhere to a patient's skin, asshown in FIGS. 28A-28C. A spacer may be placed in between the planarsurface and skin for insulation. In some variations, the planar surface2802 together with a battery 2804 and circuits 2806 may be combined intoa thin patch. This may be adhered to the skin, as shown in FIG. 28A. Inanother variations, a battery 2804 may be separate from the patch, andthe patch and battery 2804 may be configured to be separately adhered tothe skin, as shown in FIG. 28B. In yet another variation, a battery 2804and circuits 2806 may be combined, and may be configured to be adheredto the skin separately from the patch, as shown in FIG. 28C. A flexiblesubstrate configured to adhere to the patient's skin may be configuredto be worn for any suitable period of time, and may depend on theapplication. For example, in some variations the systems describedherein may be used for on-demand stimulation, and the planar surface maybe left on the patient's skin during the period of stimulation (e.g.,about one hour to about a few hours), although it should be appreciatedthat the surface may be adhered to the patient's skin for a longerperiod of time. In other variations, the systems described herein may beused for charging a battery in the implanted device. In some suchvariations, the planar surface may be left on the patient's skin forabout 1 minute to about 10 minutes, although it should be appreciatedthat the surface may be adhered to the patient's skin for a longerperiod of time.

Subwavelength Structures

As mentioned briefly above, the planar surface may be combined with oneor more subwavelength structures to form a midfield plate. A“subwavelength structure” may be defined relative to the wavelength ofthe field. If λ₀ is the wavelength in air and λ_(material) is thewavelength in a high-dielectric material, any source structure that isof dimension much less than the wavelength in air λ₀ may be termed asubwavelength structure. When the relative permittivity of thehigh-dielectric material is n, the wavelength in the high-dielectricmaterial is √{square root over (n)} times smaller than the wavelength inair, that is, λ_(material)=λ₀/√{square root over (n)}. For example, therelative permittivity of muscle at 1.6 GHz is 54, and thereforeλ_(material)=λ₀/7.3 Hence, any source structure that is of dimension onthe order of λ_(material) may be a subwavelength structure. Morespecifically, the largest dimension of each subwavelength structure dmay be in between 0.1 λ_(material) and 2 λ_(material). When this is thecase, the subwavelength structures may generate evanescent fields, andwhen they are placed in close proximity to a high-dielectric material,the evanescent fields may induce energy transfer through the propagatingmodes of the high-dielectric material. The spatial frequency spectrum ofthe output field adjacent to the source has significant components ink₀≦√{square root over (k_(x) ²+k_(y) ²)}<k_(muscle), as explained inmore detail above.

The subwavelength structures may have any suitable design configured togenerate and manipulate propagating fields in material (e.g., tissue),as described in more detail herein. In some variations, thesubwavelength structures may comprise slots in a ground plane. In othervariations, the subwavelength structures may comprise strips of metal orpatches of metal, which may be disposed over a substrate, which may inturn have a ground plane underneath, but need not. The metal strips orpatches may comprise any suitable material, such as but not limited tocopper. The metal strips and patches may have any suitable thickness,such as but not limited to about 30 μm.

Examples of suitable subwavelength structures are shown in FIGS. 7A-7B.In the variation shown in FIG. 7A, the subwavelength structure maycomprise two linear metal strips, 702 and 704, arranged end-to-end. Themetal strips 702 and 704 may be excited by a voltage source 706,described in more detail below, located between their ends. The combinedlength of the metal strips may be between about 1/10 of the wavelengthof the magnetic field in the dielectric material (e.g., tissue) andabout 2 times the wavelength of the magnetic field in the dielectricmaterial (e.g., tissue), as described above. The metal strips may beplaced on top of a planar substrate, as described above. In thevariation shown in FIG. 7B, the subwavelength structure may comprise alinear slot 710. The slot may be disposed in a planar surface asdescribed above, such as a metal plate 708 as shown, and the slot 710may be excited by a voltage source 712, to form a midfield plate(described in more detail below).

Other variations of subwavelength structures are shown in FIGS. 8A-8F.As shown in FIG. 8A, in some variations the subwavelength structure maycomprise two curved metal strips 802 and 804, which may form an arc of acircle. As shown in FIG. 8B, in some variations, the subwavelengthstructure may extend around the full arc of a circle, forming aring-shaped metal strip 806. In other variations, as shown in FIGS.8C-8F, the subwavelength structure may comprise one or more slots. Inthe variation shown in FIG. 8C, the subwavelength structure may comprisea slot 808 forming an arc of a circle. In the variation shown in FIG.8D, the subwavelength structure may comprise a ring-shaped slot 810. Inthe variation shown in FIG. 8E, the subwavelength structure may comprisetwo linear slots 812 and 814 forming a cross. In the variation shown inFIG. 8F, the subwavelength structure may comprise a linear slot 816 anda curved slot 818, intersecting at their midpoints. In each variationshown in FIGS. 8A-8F, each subwavelength structure may be excited by asingle voltage source 820 (described in more detail below). That is,there is only one point in the structure where the voltage is fixed.

Each variation of subwavelength structures described herein may produceelectromagnetic fields adjacent to the source where the spatialfrequency spectrum of these fields has non-negligible components ink₀≦√{square root over (k_(x) ²+k_(y) ²)}<k_(muscle), as explained inmore detail above. For example, FIG. 17 shows the spatial frequencyspectrum of the transverse electric field adjacent to a midfield sourcecomprising the subwavelength structure shown in FIG. 7A, where thecombined length of the metal strips in FIG. 7A is about equal toλ_(muscle) at the operating frequency of 1.6 GHz, and the source isplaced about 1 cm above an air-muscle interface. As can be seen in FIG.17, the electric field comprises non-negligible components ink₀≦√{square root over (k_(x) ²+k_(y) ²)}<k_(muscle).

In some variations, the subwavelength structures may be configured tominimize the tissue heating effect of the applied fields. Becauseelectric fields induce tissue heating, to minimize the tissue heatingeffect, subwavelength structures may be configured to yield magneticfields dominating near the source. Additionally or alternatively, thesubwavelength structures may be configured to be low profile. Forexample, it may be desirable for the subwavelength structures tocomprise slots and/or patches due to their low-profile structures.

In other variations, the subwavelength structures may be configured toyield transverse magnetic field dominating near the source. In some ofthese variations, the subwavelength structure may comprise a patchsubwavelength structure—a subwavelength metal plate on a substrateunderneath by a ground plane (as shown in FIG. 11A), a PIFAsubwavelength structure—similar to a patch subwavelength structure,except that one side of the patch may be shorted to the ground plane (asshown in FIG. 11B), a cross slot subwavelength structure (as shown inFIG. 11C), an aperture-coupled circular slot subwavelength structure,wherein the excitation of the slot structure is by a monopole inproximity to the slot but not touching the slot (as shown in FIG. 11D),and/or a half slot subwavelength structure (as shown in FIG. 11E).

FIGS. 18A-18E show the magnetic fields produced by midfield sourcescomprising the subwavelength structures of FIGS. 11A-11E. FIG. 18A showsthe magnetic field generated with a patch sub-wavelength structure asshown in FIG. 11A. FIG. 18B shows the magnetic field generated with aPIFA subwavelength structure as shown in FIG. 11B. FIG. 18C shows themagnetic field generated with an aperture-coupled circular slotsubwavelength structure as shown in FIG. 11D. FIG. 18D shows themagnetic field generated with a cross slot subwavelength structure asshown in FIG. 11C. FIG. 18E shows the magnetic field that results from ahalf slot subwavelength structure as shown in FIG. 11E. As can be seen,the midfield sources generate a magnetic field parallel to the tissueinterface, and perpendicular to the propagating wave generated in tissuethat transmits wireless power to an implanted device. In contrast, FIG.19 shows the magnetic field generated by a conventional inductivelycoupled loop source. As can be seen, the magnetic field is generatedperpendicular to the tissue interface, and is parallel with thedirection of desired wireless power transfer to an implant disposed intissue below the loop source.

Midfield Plate

The planar surface and one or more subwavelength structures as describedherein may be combined to form a midfield plate. A midfield plate maycomprise any suitable number of subwavelength structures (e.g., one,two, three, four, five, six, seven, eight, or more). Each of thesubwavelength structures may be identical, or the midfield plate maycomprise a combination of various subwavelength structures, such asthose described above.

FIGS. 9A-9B show examples of midfield plates that may comprise more thanone subwavelength structure. In the variation shown in FIG. 9A, midfieldplate may comprise two semi-circular subwavelength metal strips 902 and904, configured to form a full ring-shaped structure. Two voltagesources 906 may excite the subwavelength structures. The configurationshown in FIG. 9A is similar to the configuration shown in FIG. 8B, butthe configuration in FIG. 9A may comprise two voltage sources, creatingtwo points at which the voltages may be fixed. Thus, the configurationshown in FIG. 9A may comprise two subwavelength structures 902 and 904.FIG. 9B shows a configuration comprising a cross-shape similar to theconfiguration shown in FIG. 8F, but having four voltage sources 908,creating four points at which the voltages may be fixed, and thus, foursubwavelength structures 910, 912, 914, and 916.

In some variations, the midfield plate may comprise a combination ofmultiple subwavelength structures that to approach the performance of anoptimal source that maximizes the power transfer efficiency, asdescribed above. FIGS. 14 and 15 illustrate two such configurations formidfield sources. In the variation shown in FIG. 15, the midfield platemay comprise of an array of four configurations of subwavelengthstructures shown in FIG. 13A (described in more detail below). The fourconfigurations of FIG. 13A may be arranged in a circular configuration,with each of the four configurations rotated 90 degrees relative to itsneighbors, with the linear slots pointing toward the center of thearray. When excited, a midfield plate comprising this arrangement ofsubwavelength structures may generate circular current paths that mayapproximate the optimal current density is for power delivery across achest wall to the heart, as described above. In the variation shown, themidfield plate may be excited to form a midfield source by fourindependent radio-frequency ports connected to microstrip transmissionlines, as described in more detail with respect to FIGS. 13A-13B. Theamplitude and phase at each port may be chosen to maximize the powertransfer efficiency. For appropriate phases between the port signals,the array structure may generate circular current paths that mayapproximate the optimal current density.

If use of the midfield source shown in FIG. 15 is simulated through anapproximated chest wall as shown in the arrangement in FIG. 2A, thespatial frequency spectrum of the array along the k_(x) axis, comparedwith the theoretical optimum shown in 210 (at the skin surface) of FIG.2C, is shown in FIG. 16A. As can be seen in FIG. 16A, the evanescentspectrum may approximate the theoretical optimum, although thecontribution of the radiative modes may be about two times greater owingto the inherent directionality of the planar structure. An experimentalmeasurement of the power transfer efficiency is shows in FIG. 16B. Whentransferring power at 500 mW to an implant having a 2-mm diameter coiland submerged in a liquid solution with dielectric properties mimickingmuscle tissue, experimental studies showed that a midfield source havinga subwavelength structure configuration as shown in FIG. 15 was able toobtain efficiencies within 10% of the theoretical bound, as shown inFIG. 16B, and evidenced by a pronounced minimum in the scatteringspectrum, as shown in FIG. 16C. It should be appreciated that themidfield plate of FIG. 15 may be modified in certain ways. For example,FIG. 10B shows a midfield plate comprising a configuration ofsubwavelength elements similar to the midfield plate of FIG. 15, exceptthat the slots may comprise bends at each end. These bends may enhancesthe bandwidth of the midfield source created by exciting the midfieldplate.

Planar Immersion Lens

A solid immersion lens includes semispherical domes of high-indexmaterial placed at or near the air-material interface that allow lightto access these “forbidden” angles of refraction. This capabilityenables light to be focused to a spot much smaller than the free-spacewavelength, with a diffraction-limited resolution set by the materialwavelength ˜λ/n. Solid immersion lenses have found extensive use in manyapplications, including imaging, data storage, and lithography. Theyare, however, intrinsically three-dimensional and bulky. Replacingconventional solid immersion lenses with flat counterparts would affordopportunities for integration in complex systems, including nanophotonicchips or, in the low frequency regime, conformal biomedical devices.

FIG. 14 shows a planar immersion lens based on metasurfaces.Metasurfaces are flat devices consisting of structured arrays ofsubwavelength apertures or scatterers that provide an abrupt change inelectromagnetic properties as light propagates across the surface. Theproperties of metasurfaces can be tuned by varying the parameters of theindividual subwavelength elements to form a desired spatially varyingresponse. This freedom in design has been used to create devices thatgenerate negative refraction, light vortices, flat lensing, holograms,and other unusual interface phenomena in both optical and microwaveregimes. The planar immersion lens of FIG. 14 uses metasurfaces based onelectrically thin metallic strips with deep subwavelength spacing thatallow radiation incident from air to refract into “forbidden” angles inmaterial. This capability allows production of a thin and planar devicethat reproduces the functionality of a solid immersion lens. The devicecan be fabricated on a flexible substrate and can operate at microwavefrequencies.

The solid immersion lens is an optical tool that allows light enteringmaterial from air or vacuum to focus to a spot much smaller than thefree-space wavelength. Conventionally, however, they rely onsemispherical topographies and are non-planar and bulky, which limitstheir integration in many applications. A planar immersion lens is shownin FIG. 14. The resulting planar device, when placed near an interfacebetween air and dielectric material, can focus electromagnetic radiationincident from air to a spot in material smaller than the free-spacewavelength.

When light is focused from air into material, refraction at theair-material interface determines the diffraction limit. Conventionaloptical lenses, placed in the far-field of the interface, control onlypropagating wave components in air. As a result, their focusingresolution in material is diffraction-limited at the free-spacewavelength, λ. This is at least in part because higher wavevectorcomponents in material cannot be accessed by far-field light. These highwavevector components correspond to plane waves propagating at anglesgreater than the critical angle, which are trapped in the material bytotal internal reflection.

To allow for interface phenomena different from classical reflection andrefraction, a metasurface can be placed at or near an air-materialboundary, such as to break translational symmetry at the interface. Themetasurface can impart a phase with constant gradient ∇Φ on incidentlight, propagation is governed by a generalized form of Snell's law. Thelaw implies that radiation incident at an angle θ_(inc) refracts at aforbidden angle |θ_(ref)|>θ_(critical) if the phase gradient issufficiently large |∇Φ|>k₀−k₀ sin |θ_(inc)|. A phase gradient can beimplemented by non-periodically modulating the surface withsubwavelength structures of varying impedances. FIG. 14 shows themetasurface. The metasurface can include metallic strips with passivelumped elements (resistors, capacitors, and inductors). At microwavefrequencies, these elements can consist of patterned metal traces orcommercial impedance components. Across a resonance, the phase of thecurrent in the strips differs from that of the driving electric field bya value between [0, π]. By selecting suitable passive elements andtaking into account both the intrinsic and mutual impedances of thestructures, the spatial phase profile of the transmitted wave can beshaped within this phase range. The use of discrete passive elementsconsiderably simplifies the design as the metasurface can bereconfigured by simply changing the elements. Because coupling isexplicitly accounted for in the design, the inter-element spacing can bemade subwavelength. The phase range can be extended to the full [0, 2π]by exploiting changes in polarization (Berry phase), incorporatingelements with a magnetic response, and/or cascading multiple layers,although the limited range achieved in the immersion lens of FIG. 14using a single layer is sufficient.

Refraction at a “forbidden” angle can be achieved using the metasurfaceto create a phase gradient of ∇Φ=π/0.55λ for radiation at about 1.5 GHz.The spacing between the elements is about λ/20, and is subwavelength,such as to satisfy sampling requirements. For an s-polarized plane waveincident at θ_(inc)=30°, the beam is entirely refracted to an anomalousangle of θ_(ref)=45° that lies well beyond the critical angleθ_(critical)=30°. Because the metasurface does not rely on polarizationconversion, there is no co-polarized component refracted at an angledictated by the standard Snell's law. Unlike diffraction gratings, whichalter the spatial amplitude profile, the metasurface refracts theincident wave by modulating its phase profile and thus does not resultin unwanted diffractive orders.

Anomalous refraction still occurs when a λ/20 air gap is introducedbetween the metasurface and the interface. This effect is closelyrelated to frustrated total internal reflection. In absence of materialbelow the metasurface, the incident wave completely reflects off themetasurface and forms an evanescent wave at the surface. This evanescentwave propagates along the surface in the direction of the phasegradient, which is a behavior that cannot be realized with a grating.When the material is placed in close proximity to the metasurface, theevanescent field phase matches to a propagating wave in the material,allowing the incident beam to tunnel into the material with the nettransport of energy across the interface. By varying the angle ofincidence relative to the phase gradient, the angular spectrum of thetransmitted beam can be made to lie almost entirely in the forbiddenregion. The results appear to be in agreement with the generalizedSnell's law. The spread around the predicted angle is due at least inpart to finite size of the aperture.

To design a planar immersion lens, a field source in air that focuses toa kin spot in material can be found. Although focusing across a planarinterface has been previously studied, classic expressions for theoptimal field source consider only far-field light and yield a ˜λ focalspot. A more general approach can be used that accounts for evanescentwaves at the interface. An optimization problem over the space ofcurrent sheets j_(s) in the source plane (taken to be z=0) isformulated. The solution can be defined to be the current sheet thatmaximizes a metric for the degree of focus. Assume that the material isdissipative, allowing small but non-zero loss. The efficiency of workperformed on the material as the focusing metricη=∝″|E(r _(f))|² /∫dr∈″|E(r)|²   Equation 1where r_(f) is the focal point, ∝″ the imaginary part of thepolarizability of the object at the focal point, and ∈′″ the imaginarypart of the material dielectric permittivity. α can be set to be thepolarizability of a “virtual” sphere centered at the focal point: thesphere has the same dielectric permittivity as the background materialand can be made arbitrarily small (e.g., the diameter of a computationalmesh unit). The electric field, E, can be found by propagation from thecurrent sheet js as described by the Green's function G(r, r′).

The optimization problem can now be considered in the operatorformalism. Using Dirac bracket notation, E and j_(s) can be representedrespectively as functions |ψ> and |φ> in Hilbert space. They are relatedthrough the operator expression |ψ>=Ĝ|φ> where Ĝ is the Green's functionoperator. A focal position operator {circumflex over (Φ)} can be definedsuch that the numerator in Equation 1 can be written as<ψ|{circumflex over (Φ)}ψ>=∝″∫drδ(r _(f))|E(r)|² =|E(r _(f))|².  Equation 2

Similarly, a power loss operator {circumflex over (Σ)} can be defined toyield

$\begin{matrix}{\left\langle {\psi{\hat{\sum}}\psi} \right\rangle = {{\int{\mathbb{d}r}} \in^{''}{{{E(r)}}^{2}.}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

Optimal focusing occurs when the choice of the source current density|φ> maximizes Equation 1. Focusing can thus be posed as an optimizationproblem

$\begin{matrix}{\max\limits_{{{\phi}\rangle} \in S}\frac{\left\langle {\phi{{{\hat{G}}^{\overset{.}{T}}\hat{\Phi}\hat{G}}}\phi} \right\rangle}{\left\langle {\phi{{{\hat{G}}^{\overset{.}{T}}{\hat{\sum}\hat{G}}}}\phi} \right\rangle}} & {{Equation}\mspace{14mu} 4}\end{matrix}$where S is the set of all current sheets on a plane above the z=0 plane.The form of Equation 4 is a generalized eigenvalue problem involving theoperators Â:=Ĝ^({dot over (T)}){circumflex over (Φ)}Ĝ and

$\hat{B}:={{\hat{G}}^{\overset{.}{T}}{\hat{\sum}{\hat{G}.}}}$solution is given by the two-dimensional current density that satisfiesÂ|φ_(opt)>=λ_(max){circumflex over (B)}|φ_(opt)> where λ_(max) is thelargest generalized eigenvalue. If {circumflex over (B)} is invertible,then the solution |φ_(max)> can be obtained from a standard eigenvaluedecomposition of the operator {circumflex over (B)}⁻¹Â. Numericalcomputation can be considerably accelerated by (i) selecting the planewave basis, which diagonalizes the Green's function operator for themultilayer geometry, and/or (ii) exploiting degeneracies due toazimuthal symmetry about the focal axis. The calculation reduces toinversion of dyads at each spatial frequency, without need to explicitlyform the full system matrices. This inverse filtering process is closelyrelated to time-reversal and can be generalized to transparent media byallowing the material loss to asymptotically approach zero.

Consider a two-dimensional geometry where the material has a refractiveindex n=2. For incident s-polarized radiation, Equation 4 can benumerically solved to obtain a source that focuses to a line at a 4λ/ndistance. A linear metasurface can be used to shape a normally incidentplane wave such that the exiting field matches the solution. Therequired impedance values of the passive elements can be solved by usinga point-matching method. The line width of the focal spot cansubwavelength, such as 0.42λ full-width half-maximum (FWHM). To verifythat the focusing effect is due to phase (not amplitude) modulation ofincident wave, the passive elements can be removed such that the surfaceacts as a grated aperture. The focal spot for the grating is notsubwavelength (˜λ); the intensity at the focal point is also decreasedby a factor of four when the passive elements are removed. The physicsunderlying the lensing effect is substantially different from near-fieldfocusing devices. Unlike near-field plates, which focus evanescent wavesat a strictly subwavelength distance (typically less than λ/10), theimmersion lens' focusing ability results, at least in part, fromconventional interference between propagating waves and, as a result,the focal plane can be many wavelengths away. The enhanced resolution ofour lens follows from the shaping of the near-field phase profile, whichcouples the incident wave to forbidden angles on interaction withmaterial, rather than the near-field interference effects of near-fieldplates. Because the focusing is not subject to the intrinsic decay ofthe near-field, the intensity at the focal spot can be comparable to orhigher than the incident intensity. As with solid immersion lenses, thefocusing resolution remains subject to the diffraction limit, althoughthe spot size is a function of the material rather than the free-spacewavelength.

Next, consider a three-dimensional geometry where a planar source ispositioned a subwavelength distance (λ/15) above a material whoserefractive index at microwave frequencies approximates biological tissue(real part n=8.8). Due to symmetry about the focal axis, thepolarization of the fields at the focal point can be arbitrarilyspecified. Setting the electric field to be linearly polarized in the xdirection, the solution to Equation 4 can be a surface wave consistingof concentric ring-like currents around the focal axis. In air, theresulting fields are evanescent and non-stationary, propagating in-planetowards the focal axis. The intensity profile at the source plane issignificantly non-zero only within a finite circular region. The radiusof this region defines an effective aperture size that is directlyrelated to the loss in the material system and the depth of focus. Atthe focal plane in the material, the field converges to a spot of widthλ/11, measured FWHM, at a distance of about 2.3λ/n (wavelength inmaterial) from the source plane. Although the wave originates in air,the spot size approaches Abbe's diffraction limit λ/(2n sin θ_(ap)) inhomogenous material, where θ_(ap) is the half-angle the aperturesubtends the focal point, due to the source's ability to accessforbidden wave components.

In sum, FIG. 14 shows a midfield plate with the functionality of a solidimmersion lens. The enhanced focusing resolution of the device results,at least in part from the ability of metasurfaces to control thenear-field with subwavelength resolution on interaction with dielectricmaterial. At optical frequencies, planar immersion lenses can beimplemented with closely-spaced plasmonic antennas or dielectricresonators, with mutual interactions accounted for by tuning theproperties of optical “lumped” elements. By incorporating subwavelengthstructures that interact with the magnetic field component of incidentradiation, the metasurface could also modify the optical impedance,allowing reflection at the interface to be eliminated. As thefabrication of the metasurface is simple and planar in nature, themetasurface-based lens can be integrated into complex systems, such asnanophotonic chips or conformal biomedical devices.

In the variation shown in FIG. 14, the midfield plate may comprise of anarray of curved sub-wavelength structures 1402, 1404, 1406, 1408, 1410,1412, 1414, and 1416, and a subwavelength dipole 1018. These may benested to form a bulls-eye-like pattern, as shown. The choice of thestructures is configured to produce propagating fields converging to asubwavelength spot in tissue, as described in more detail above. Inother variations, the midfield plate may comprise slots instead of metalstrips, having the same configuration as shown in FIG. 14. FIG. 10Ashows a top view of the midfield source in FIG. 15. Because the midfieldsource of FIG. 14 comprises more excitation ports than the midfieldsource of FIG. 15, it may be more invariant to the characteristics ofthe tissue in between the source and an implant, although the midfieldsource of FIG. 15 may work well in a wide range of materials.

Excitation Ports

The midfield plates described herein may be configured to manipulateevanescent fields produced by a power source. In some variations, thesubwavelength structures of the midfield plates may be excited byexcitation ports, as described briefly above. In some variations inwhich the midfield plate comprises more than one subwavelengthstructure, each subwavelength structure may be excited by a separateexcitation port. In other variations in which the midfield platecomprises more than one subwavelength structure, a single excitationport may excite more than one subwavelength structure.

The excitation port may comprise a radio-frequency port. Aradio-frequency signal may be generated by a signal generator (e.g., anoscillator). The radio-frequency signal may have any suitable frequency.In some variations, the frequency may be between about 800 MHz and about1 GHz. In other variations, the frequency may be between about 2.3 GHzand about 2.5 GHz. As described above, the optimal frequency forefficient power transfer may depend on the material located between themidfield source and the receiver coil. For example, in the example of achest wall structure, the optimal frequency may be about 1.6 GHz. Insome variations, the frequency of the signal may be adjustable.Adjusting the operating frequency of the source may allow for adjustmentof the power received by an implant, and/or may allow the midfieldsource to be used for implants located within different materials and atdifferent locations within materials.

In some variations comprising more than one excitation port, theradio-frequency signal may be divided into multiple radio-frequencysignals, for example, using a power divider (e.g., a Wilkinson powerdivider) on a control board. In some variations, the radio-frequencysignal may be divided symmetrically into each of the multipleradio-frequency signals, but need not be. It should also be appreciatedthat rather than dividing a single radio-frequency signal into multipleradio frequency signals, the device may comprise multiple signalgenerators. Each radio-frequency signal may be transmitted via cables(e.g., semi-rigid coaxial cables) from the control board to eachradio-frequency port. The signals may additionally or alternatively befed through a phase shifter (e.g., analog 400°, +3.5/−2.0° error) withcontrollable phase and/or then amplified (e.g., gain 14 dB). This mayproduce controlled phase and amplitude signals at each radio-frequencyport.

Two example architectures for multiple excitation ports are shown inFIGS. 12A-12B. In the architecture shown in FIG. 12A, the signalgenerated by the signal generator 1202 may be divided, and then eachdivided signal may be fed through an attenuator 1204, which may havevariable controllable attenuation settings. The signals may then be fedthrough phase shifters 1206 and amplifiers 1208. The architecture shownin FIG. 12B may be able to produce the same controlled phase andamplitude signals, but with fewer components, by combining the amplifierand the amplitude control element into a single component 1210. FIGS.21A-21B, described in more detail below, also shows another schematic ofa similar architecture for multiple excitation ports 2110.

In some variations in which the midfield source comprises more than onesubwavelength structure, two or more subwavelength structures may beexcited by a single excitation port. That is, the excitation for eachsubwavelength structure may not be independent. This may be accomplishedby conveying the signal from one radio-frequency port to multiplesubwavelength structures, for example, via a microstrip. For example,FIGS. 13A-13B show an example of a source having subwavelengthstructures similar to those described with respect to FIG. 9B, having alinear slot 1302 and a curved slot 1304 intersecting near theirmidpoints, wherein the four subwavelength structures described above areexcited using the same single excitation port 1306. A ring shapedmicrostrip transmission line 1308 may be located underneath a groundplane 1310, separated by a dielectric (e.g., air or a substrate 1312).Each point on the microstrip transmission line 1308 may have a differentphase. By adjusting the dimension of the microstrip transmission line1308, multiple subwavelength structures (e.g., four subwavelengthstructures as shown) may be excited with excitation port 1306.

While the signal generators described above are voltage sources, inother variations, one or more subwavelength structures may be excited bya current source. In yet other variations, the voltage source or currentsource may be replaced by a reactive element such as a resistor, acapacitor, an inductor, or a combination of these elements. In thesevariations, the subwavelength structure may be excited by a plane waveor a waveguide. As such, the ratio of the voltage to current may befixed at the location of the reactive element instead of having a fixedcurrent or a fixed voltage.

Real-Time Dynamic Focusing

In some variations of the midfield sources described here, the focalregion may be dynamically shifted without mechanical reconfiguration ofthe source, using the degrees of freedom provided by the amplitudes andphases of the input port signals. This may be useful in clinicalapplications in which the source may be used to power implantabledevices configured to interact with organs in rhythmic motion (e.g., dueto breathing or heartbeat), or implantable devices configured to moveinside the body. In order to shift the focal region, the excitation ofindividual subwavelength structures may be reconfigured in real-time,enabling various field patterns to be synthesized, including those withspatially shifted focal regions. FIG. 20 illustrates field patterns withspatially shifted focal points designed by adjusting the relative phasesbetween the port signals. The upper diagrams in FIG. 20 show formationof a propagating wave in a direction directly below the source. Thelower diagrams in FIG. 20 show adjustment of the focal point.

FIGS. 21A-21B show a possible architecture for a controller of amidfield source comprising four excitation ports. As shown in FIG. 21A,a radio-frequency signal may be brought from a signal generator 2102 andmay be divided symmetrically into multiple radio-frequency signalsthrough a power divider 2104, such as a Wilkinson power divider.Following power division, the signals may be connected to parallelstages for variable attenuation, phase shifting via elements 2106, andamplification via elements 2108. This may produce controlled phase andamplitude signals at each excitation port 2110. In other embodiments,following power division, the signals may be connected to parallelstages for phase shifting and variable amplification.

The phase shifters 2106 and 2108 may be controlled by a control unit2112. In some variations, a “greedy” phase search algorithm may be usedto change the phase and/or magnitude settings in each element of themidfield coupler so as to dynamically shift the focal region. In somevariations, the algorithm may be based on closed-loop feedback, whichmay be relayed over an optical fiber 2114, as shown in FIG. 21B. Inother variations, the implant may comprise a wireless transceiver, whichmay enable an untethered realization of closed-loop feedback and otherrelated control algorithms. For example, the feedback may be based ondetected power levels of received wireless energy by the implant, asdescribed in more detail below. Based on the power measurement feedbackfrom the implant module, the adjustments may be made automatically andin real time to optimize wireless power transmission between the sourceand the implant.

An example of the effect of such an adaptive algorithm is shown in FIGS.22A-22E. To generate the images shown there, the implant 2216 comprisingan LED, shown in FIG. 21B, was moved in an “S” shaped trajectory withina liquid solution whose properties mimicked muscle tissue, asillustrated in FIG. 22A. FIG. 22B shows the strobed position of the LEDwhen a real time control algorithm, such as one described above,dynamically tracked for motion. FIG. 22C shows the strobed position ofthe LED without dynamic focusing. As can be seen by comparing FIGS. 22Band 22C, the field pattern is static and focused at the center in thenon-adaptive case as compared to the adaptive case. Over the “S” shapedtrajectory of motion, the adaptation eliminated dark regions thatoccurred in the static case, indicating a coverage area much wider thanthat intrinsic to the focal region. The effect of the adaptive algorithmcan also be seen in FIGS. 22D and 22E. FIG. 22D shows the power receivedby the implant, as measured by the flashing rate of the LED. As can beseen, the dynamic phase adaptation algorithm enables higher levels ofpower to be transferred as the device moves. FIG. 22E shows the phase ofeach port, relative to a phase stationary port 4, controlled by thealgorithm as the implant is in motion.

Internal Module (Implant)

Also described here are implants that may be configured to receive powerfrom a midfield source as described herein. In some variations, theimplants may be configured to provide stimulation (e.g., electricalstimulation) to a target site (e.g., a targeted nerve, muscle, or tissueregion), described in more detail below. Additionally or alternatively,the implants may be configured to perform a sensory function at a targetsite, as described in more detail below.

Midfield sources may yield a highly oscillatory electric current densitythat may force the output field to converge on a subwavelength spot,creating a high-energy density region deep in tissue. Inside thisregion, a power-harvesting structure in an implant may be able to bemade extremely small. Because the system may operate in the midfieldregion, the implant may mostly harvest energy from the transverseelectromagnetic fields (i.e., the oscillation direction of the field isperpendicular to the direction of propagation). This is different fromnear-field coupling systems, where implants may mostly harvest energyfrom the axial electromagnetic fields (i.e., the oscillation directionof the field is in parallel to the direction of propagation). The focalregion from the midfield sources may be dynamically shifted using thedegrees of freedom provided by the amplitudes and phases of the inputport signals, as described herein. The implant may incorporatecomponents for received power sensing and wireless communications toenable the realization of real-time dynamic focusing, as describedherein.

An implant may comprise of a power receiver, data transceiver, and/orstimulation and sensing components. In some variations, the powerreceiver may comprise of a coil and one or more AC-DC conversionbranches for different output voltage requirements. The data transceivermay comprise of a data receiver, data transmitter, multi-accessprotocol, and/or an identification, and a digital controller. Thestimulation and sensing components may comprise current drivers for bothelectrical and optical stimulations, sensing frontends for electricalsensing, electrodes, and/or LEDs (light emitting diodes). Thesecomponents are described in more detail below, but should be appreciatedthat the implant need not comprise all of these components.

Size and Shape

The implants may have any suitable shape and dimensions. In somevariations, the implants described here may be configured to fit insidea delivery device, such as but not limited to a catheter or hypodermicneedle. In these variations, the implant may be able to be injected intoa target site (e.g., a targeted nerve or muscle region) directly withoutthe need for leads and extensions. FIG. 23A shows a photograph of anexample of an implant 2302 as described here, on a human finger. FIG.23B shows the same implant 2302 before epoxy encapsulation next to a10-French (˜3.3 mm) catheter sheath for size comparison. FIG. 23C showsa photograph of the same implant 2302 inserted in the lower epicardiumof a rabbit via open-chest surgery. In the variation shown in FIGS.23A-23C, the implant is a wireless electro-stimulator 2 mm in diameter.

In some variations, the implants may have a cylindrical,semi-cylindrical, circular, or rectangular shape, or the like. In somevariations the implants may have a diameter (or greatest cross-sectionaldimension) between about 10 μm and about 20 mm, between about 100 μm andabout 10 mm, or between about 1 mm and about 5 mm. More specifically, insome variations the implants may have a diameter (or greatestcross-sectional dimension) of about 2 mm. It should be appreciated thatin other variations, the diameter (or greatest cross-sectionaldimension) of the implants may be greater than 20 mm. In somevariations, the implants may have a height of between about 10 μm andabout 20 mm, between about 100 μm and about 10 mm, or between about 1 mmand about 5 mm. More specifically, in some variations the implants mayhave a length of about 3 mm. It should be appreciated that in othervariations, the length of the implants may be greater than 20 mm.

In some variations, the implant may be encapsulated in a suitablematerial. For example, in some variations, the implant may beencapsulated in epoxy. In other variations, the implant may beencapsulated in ceramic or glass. in some variations may compriseanchors or other structures to help secure the implant in place. In somevariations, the electrodes may be configured to be used fixation, inaddition to stimulation and/or sensing. For example, the electrodes maycomprise barbs, as shown in FIG. 23A. In other variations, theelectrodes may comprise a screw shape, which may be able to be screwedinto tissue. In other variations, the implant may comprise fixationstructures that are not also electrodes. For example, the implant maycomprise loops or hooks. Such implants may be able to be fixed to tissueby suturing the adjacent tissue through the loops or hooks.

Coil

The coil may be configured to receive energy from a source (e.g., aspatially adaptable electromagnetic field generated by the sourcesdescribed herein). The energy may be received by the coil asmagnetization, due to induced current in the coil. The coil may compriseany suitable material, such as but not limited to copper, gold, oraluminum. The coil may comprise any suitable number of turns. The numberof turns may depend on the frequency of the midfield source. In somevariations, the coil may comprise between about 1 turn and about 15turns. FIG. 23B shows an example of a coil 2304 comprising a multi-turncoil structure comprising 200 μm diameter copper wire wound with aninner diameter of 2 mm.

AC-DC Conversion and Charge Pump

In some variations of implants comprising an AC-DC power conversionmechanism, the AC-DC power conversion mechanism may comprise rectifyingcircuitry. The rectifying circuitry may be configured to convert energy(e.g., a spatially adaptable electromagnetic field generated by thesources described here) received by the implant (e.g., by a coil asdescribed above) to a DC signal. In some variations, the AC-DCconversion circuitry may be divided into low-voltage and high-voltagedomains. This may increase the efficiency of rectification and powermanagement of wirelessly powered implants operating in anelectromagnetically weakly coupled regime. In some variations, theimplants may comprise a charge pump. In one variation, two diodes (e.g.,Schottky diodes) and two capacitors (e.g., 10 nF capacitors) may bearranged in a charge pump configuration. At low frequencies, anadditional capacitor may be used in order to match the impedance of thecoil and the rectifier. A charge pump and flash control integratedcircuit may be placed after the rectifier for up-converting therectified voltage.

Integrated Circuit

In some variations, the implants described here may comprise anintegrated circuit. For example, as shown in FIG. 23B, the coil 2304 maybe disposed over and connected to an integrated circuit 2306. In somevariations, the integrated circuit may be configured to regulate pulseamplitudes. In some variations, the implants may comprise non-volatilememory. For example, the implants may comprise flash memory, which maybe configured to record data such as usage information (e.g., the timeof activation and setting of the current deriver), and/or to storemeasurements from sensors (described in more detail below). In somevariations, each implant module may have its own identification tag,such as an identification tag stored in the memory of the implant. Insome variations, the implant may comprise a digital core, which may beconfigured to coordinate the interaction among various components of theimplant, communication between the implant and external components, andthe multi-access protocols, as described below.

Energy Storage Component

In some variations, the implant may comprise an energy storagecomponent. For example, the implant may comprise a rechargeable battery.The rechargeable battery may be configured for temporary energy storage,and/or for use as an efficient charge pump for power managementcircuitry. In some variations, the rechargeable battery may comprise athin film battery. In some of these variations, the thin film batterymay be stacked, which may allow for increased energy density. In someother variations, the rechargeable battery may comprise a lithiumbattery. Energy storage components may enable the implant to be operatedwithout continuous coupling to an external power source as describedherein. The external power source may be used to charge the implant,which in some variations may be able to be charged with only a fewminutes to tens of minutes of wireless charging per week or month.

Sensors

Because the power levels deliverable to the implants by power sources asdescribed herein may exceed requirements for microelectronictechnologies, more sophisticated functions may be implemented, such asreal-time monitoring of chronic disease states or closed-loop biologicalsensing and control by the implant. In some variations, the implant maycomprise one more sensors. In some variations, the sensors may include,for example, temperature sensors. In other variations, the sensors maycomprise optical sensors and/or imaging devices. In yet othervariations, the implants may comprise chemical, pressure, oxygen, pH,flow, electrical, strain, magnetic, light, or image sensors. In somevariations, the sensors may allow the depth at which the device isoperating to be determined. In some variations, the sensors may compriseone or more electrodes.

In variations in which the implant comprises one or more sensors, theimplant may comprise one or more pre-amplifiers, analog-to-digitalconverters, and/or drivers for the one or more sensors. In variationshaving analog-to-digital converters, the analog-to-digital convertersmay be used to discretize signals from pre-amplifiers. In somevariations, the output signals from the analog-to-digital converters maybe stored in non-volatile memory of the implant (as described in moredetail above), or in other variations may be sent to the source or otherexternal component via a radio-frequency modulator (as described in moredetail below).

In variations in which the implant comprises one or more sensors, thesensors may be configured to provide feedback to the source or to auser. For example, in some variations the implant may comprise one ormore sensors configured to detect the instantaneous power level receivedby the implant. This information may be sent via a data transmitter(described in more detail below) to the source. This may allow foradaptive focusing of the focal region of the field, as described in moredetail herein. In other variations, the information from the sensors maybe provided to a user, for example via a user interface such asdescribed herein. In some variations, the data may be further analyzedor stored outside of the implant. Information from the sensor or sensorsmay, for example may allow for wireless real-time monitoring,diagnosing, and/or treatment of patients.

Stimulation

In some variations, the implants described herein may be configured todeliver a stimulus to tissue. The stimulus may be any suitable type,such as but not limited to electrical, optical, chemical (e.g., theimplant may be configured for drug delivery), or mechanical. Invariations in which the implants are configured to deliver an electricalstimulus, the implant may deliver the electrical stimulus via one ormore electrodes. The implants may comprise programmable current drivers,which may allow for a range of stimulus parameters (e.g., for electricalstimuli, intensity, duration, frequency, and shape) to be delivered. Insome variations in which the system comprises a user interface asdescribed herein, the programmable current drivers may be programmed viathe user interface (e.g., via a wireless data link to the implant). Asshown in the variation in FIGS. 23A-23C, in some variations the implantmay comprise two electrodes 2308. The two electrodes 2308 may be locatedon one side of the implant 2302, such that they may be inserted intotissue, as shown inserted in the lower epicardium of a rabbit in FIG.23C.

FIG. 24A shows a circuit schematic for a power receiver (an implant).FIG. 24B shows a circuit schematic for a data transceiver. FIG. 24Cshows a circuit schematic for a stimulator and sensor.

FIGS. 25A-25C show circuit schematics for an implant that may beconfigured to stimulate tissue (e.g., an implant configured as apacemaker). FIG. 25A illustrates a lumped circuit model of the receiver.As shown, the AC voltage V_(C) generated across the coil 2502 by thesource (as described in more detail herein) is converted to DC powerthrough the rectifier circuitry 2504. FIG. 25B illustrates theequivalent circuit at the nth reference power level when the circuit isat resonance. FIG. 25C illustrates the detail of the circuit components.As shown, the implant may comprise a rectifier, a charge pump, a pulsecontrol integrated circuit, a storage capacitor, and an LED. In othervariations, the LED may be replaced by a pair of electrodes, which maybe configured for electrostimulation. In some variations, the rectifiercircuits may comprise with two diodes and two capacitors arranged incharge pump configuration. A charge pump may be placed after therectifier for upconverting the rectified voltage, for example, from 0.7V to 2 V, necessary to drive the LED or electrodes. Charge may betemporarily stored on a capacitor. A pulse control unit may be used tocontrol the pulse frequency and width. In the example shown in FIG.25A-25C, the circuitry also included a LED configured to encode thepower flowing through the pulse control circuitry 2506 via its flashingfrequency. The non-linear properties of the rectifier and pulse controlunit may enable the unknown parameter R_(C) to be estimated if thecircuit is characterized at two reference flashing frequencies. Once theenvironment-dependent parameter R_(C) is known, the transferred powermay be estimated. In some variations, the LED in FIGS. 25A-25C may bereplaced by a pair of electrodes, which may be configured to stimulatetissue and/or nerves.

Data Transmission

The implant may be capable of wireless data transmission, and in somevariations may comprise a wireless data link between the implant and themidfield source or another external component (e.g., an external userinterface). The wireless link may be unidirectional or bidirectional,and thus, in some of these variations the implant may comprise a datareceiver. The wireless link may be configured to activate the implant(e.g., activate stimulation in variations comprising stimulators),remotely program or configure the implant (e.g., adjust implantsettings), and/or receive data from one or more sensors.

The data transmitter of the implanted module may use pulsedradio-frequency modulation. In some variations, radio-frequencymodulation may be desirable because conventional load modulation may notwork in the midfield due to the low quality factor of the implantantenna, which may lead to poor signal-to-noise ratio and substantiallink margin fluctuation. In other variations, non-coherent modulationtechniques such as amplitude shift keying and frequency shift keying maybe used for ease of implementation. To ease detection at the externalmodule, the data and power carriers can operate at different centerfrequencies. In some variations, the implants may utilize multi-accessprotocols, which may coordinate the functions of the implant (e.g., suchas coordinating multi-site stimulation). In some variations, themulti-access protocols may utilize time multiplexing and frequencymultiplexing.

The data rate for the wireless data link may be any suitable rate (e.g.,from a few kbps to 10's of Mbps). For example, in some variations thedata rate for the downlink from the source to the implant may be a fewMbps or lower, while the data rate for the uplink from the implant tothe source may be higher, such as in the range of 10's of Mbps orhigher.

User Interface

In some variations, the systems described here may comprise a userinterface, which may be used, for example, by a clinician or patient.Some variations of the user interface may be integrated with the powersource, while in other variations the user interface may be separatefrom the power source. In some variations in which the user interface isseparate from the power source, the user interface may comprise a mobilecomputing device, such as a smartphone, tablet, or wearable computer. Inthese instances, the system may comprise a wireless or wiredcommunicating link, which may allow for bidirectional communicationbetween the power source and/or the implant and the mobile computingdevice. This may allow a patient and/or clinician to interface (e.g.,receive or input information) with the power source or implant using thedisplay of the mobile computing device.

While the source, implant, and user interface are described here as asystem, it should be appreciated that the devices described herein maybe used alone or in combination with other devices and systems. Itshould also be appreciated that the systems described here may beconfigured based on the particular needs or requirements of the enduser. For example, the implants may have a modular design and may bemodified to include those components desirable for the intended use. Insome variations, all the above building blocks in the implanted modulemay be integrated into a single die as system-on-chip (SoC) or multipledies enclosed in a single module as system-in-package (SiP).

Methods/Applications

Also described herein are methods of wirelessly powering implants, suchas those described herein, using midfield sources as described herein.The implants described herein may be implanted in any suitable location.In some variations, they may be implanted through minimally invasiveprocedures, such as via catheter or hypodermic needle. The implants maybe implanted in humans or in other animals such as pets, livestock, orlaboratory animals such as rabbits, mice, rats, or other rodents. Theimplants may be used for any number of applications, such as but notlimited to muscular stimulation, stimulation/sensing to regulate apatient's heartbeat, deep brain stimulation, drug delivery, and/orbiological, physiological, and chemical sensing.

The midfield sources described herein may be used to transfer power tothe implant. In some variations, the power received by the implant maybe adjusted by adjusting an operating frequency or other parameter ofthe midfield source, as described in more detail above. Additionally oralternatively, the parameters may be adjusted in real time to modify thefocal region of the midfield source, in order, for example, to trackmovement of the implant.

For example, in some variations the systems described herein may be usedfor cardiac pacing. In some of these examples, one implant may bedelivered into the right ventricle of a patient, while a separateimplant may be delivered to the left ventricular epicardium. Theimplants may be delivered in any suitable manner, such as via cathetersthrough the vasculature (e.g., the implant may be delivered to the leftventricular epicardium via the coronary sinus and coronary vein). Theseimplants may comprise both stimulation and sensing electrodes, which maybe configured to apply leadless pacing to the heart. The systemsdescribed herein may thus allow leadless biventricular pacing to beachieved with only minimally invasive procedures. This may substantiallyreduce procedure time and complications.

An example of wireless cardiac pacing using a system as described hereis shown in FIG. 23C, described in more detail above. After implantationof a wireless electro-stimulator into the lower epicardium of a rabbitas shown in FIG. 23C, the chest was closed. The electro-stimulatordevice shown in FIG. 23C was about 2 mm in diameter, weighed about 70mg, and as capable of generating 2.4 μJ pulses at rates dependent on theextracted power. Its characteristic dimension was at least an order ofmagnitude smaller than existing commercial pacemakers due to the absenceof a battery. A portable, battery-powered midfield source shown in FIG.29 was positioned about 4.5 cm (about 1 cm air gap, and 3.5 cm chestwall) above the implant 2302. The midfield source was used to coupleabout 1 W of power into the chest, and the operating frequency of thesource adjusted to the estimated resonant frequency of the circuit andmaintained for several seconds, as shown in FIG. 23D. Cardiac activityof the rabbit was monitored by ECG, shown in FIG. 23E. The cardiacrhythm could be controlled in a fully wireless manner by adjusting theoperating frequency. When the operating frequency was coincident withthe resonant frequency of the circuit, the pulse amplitudes weresufficient to pace the heart, as indicated by the increased rate andregularity of the ECG signal in FIG. 23E. FIG. 23F shows theautocorrelation function of both the off-resonance and resonant sectionsof the ECG signal. Peaks in the autocorrelation function are marked withsquares.

Similar methods may be applied for any other optical or electricalstimulation task in the body. For example, similar methods may be usedto stimulate neurons or muscle cells. For example, similar systems andmethods may be used for deep-brain stimulation. Current procedures fordeep-brain stimulation involve drilling holes with diameters of over 1cm in the skull to insert a lead and the extension from the lead to thestimulating module. Due to the invasiveness of the procedure, only alimited number of target sites are generally selected for placing theelectrodes. In addition, the leads may not be MRI compatible. Bycontrast, the implants described herein for use with midfield sourcesmay be injected into the brain via other less invasive routes, andrequire no lead or extension wire. This may allow for more target sitesfor stimulation and may be MRI safe. Moreover, multiple devices may beimplanted and used for stimulation in a synchronized manner.Additionally, the use of systems as described herein may result in lessinfection and lower regulatory risk.

As another example, the systems and methods described herein may be usedfor spinal cord stimulation. Batteries in newer models of spinal cordstimulator are rechargeable due to the high power requirement. However,their powering approaches rely on inductive coupling (or near-fieldcoupling). Since the harvesting components are large in these systems,they can only be placed subcutaneously and not deeper. Therefore, thelead and extension wires in these systems may potentially restrict thelocation of the electrodes for effective stimulation. Lead dislodgementand infection may be major sources of complications. Because theimplants described herein are much smaller, the entire implant may beplaced next to the targeted nerve region in the spinal cord and may notrequire a lead wire. Again, this may result in less infection, lessdamage to the spinal cord tissue, and more effective stimulation.

As yet another example, the systems and methods described herein may beused for peripheral nerve stimulation. Most current devices supportlow-frequency stimulation, and only a few support high-frequencylow-intensity stimulation, due to the much higher power requirement. Thesystems described herein may be able to support both modes. In addition,the bidirectional wireless link as described herein may provide theability to switch between different modes, and to personalize thestimulation paradigms to individual patient.

As mentioned above, the systems and methods described herein may also beused for stimulating muscle cells. For example, the systems and methodsdescribed herein may be used to treat obstructive sleep apnea (OSA). Theimplants described herein may be able to be injected and directlyembedded into the muscular tissue near the tongue, and may then be usedto deliver electrical stimulation to open the airway of a patient duringsleep. Multiple implant modules may be injected into different musculargroups to intensify the muscle contraction. When needed, patients may beable to charge the implants with the midfield source. Additionally oralternatively, the data transmission capabilities may allow for downloada time stamp of each OSA episode, which may be able to be sent to aclinician. The implants may also be able to be reprogrammed withoutremoval. In some cases, the reprogramming may be based on the datacollected.

When the implants described herein are used to stimulate excitablecells, in some variations they may be used for temporary treatmentapplications, in which implantation of a long-term implant isundesirable. For example, currently, screening tests are typicallyperformed before a permanent impulse generator is implanted. During thescreening test, a patient may receive a temporary, external impulsegenerator. The generator may connect to an extension and a lead, whichmay be surgically placed in the body. In this period, the externalimpulse generator collects patient usage data and efficacy of thetreatment. However, an implant as described herein may be injected intothe targeted nerve/muscle region, eliminating the need for a temporarygenerator with leads. In addition, the implants described herein may beused instead of temporary sensing and pacing leads in patients aftercardiac surgery.

The systems and methods described herein may also be used forapplications other than stimulation of excitable cells. For example,they may be used in medical sensing applications. Battery less implantedsensors are typically passive in nature—that is, there is no activecircuitry in the device to condition the sensed signals due to the lackof an efficient wireless powering approach. To compensate for the poorsignal quality, a sophisticated and large external reader is generallyrequired. The passivity of the sensors may also limit the stimuli thatmay be detectable. The midfield sources and implants described hereinmay allow for the transfer of a substantial amount of power to smallimplanted modules at nearly any location in the body from a palm-sizeexternal module. This enables an array of new sensing applications forcontinuous monitoring, for example, post-surgery oxygen sensing in theheart and the brain.

As another example, the systems and methods described herein may be usedfor wireless endoscopes. Current capsule endoscopes have limited batterylifetime, sometimes leading to incomplete small-bowel examination. Thislimitation may be addressed by the systems described here. In addition,since the implants described here may be able to be significantlysmaller than current capsule endoscopes, patients may be able to swallowmultiple devices simultaneously. Each device may orient differently inthe intestine, and therefore may take images from different angles atthe same location, improving the field of view, allowing for improveddiagnosis. Finally, the probability of retention may be reduced,avoiding the need for surgical or endoscopic retrieval.

The systems and methods described herein may also be used for implanteddrug delivery. Current implanted drug delivery systems are large and aregenerally cannot be placed sufficiently close to the site at which thedrug is needed. An implant as described here may further comprise one ormore drug reservoirs. The implant may be injected or delivered viacatheter to a target tissue region (e.g., a tumor). The drug reservoirsmay be activated to release drug by the midfield source. In somevariations, the activation may be controlled by a patient or clinicianvia a user interface, as described herein.

The systems and methods described herein may also be used in laboratoryexperiments with lab animals, such as rodents (e.g., mice, rats, etc.).The small size of the implants may allow for monitoring capabilities notpreviously available or easily implemented. For example, the implants asdescribed herein may be used to monitor or sense parameters and/orprovide stimulation. The implants may be, for example, implanted on ornear the brain of a rodent to monitor electrical signals. The implantcan be wirelessly powered with the midfield source described above, andmay be able to be configured to communicate information back to theexternal module.

EXAMPLE

A system as described herein was used in two simulations of powertransfer to an implant, using porcine tissue volumes: a first simulatingplacement in the left ventricle of the heart, and a second simulatingplacement in the cortex region of the brain. The source and implant werelocated at least 5 cm apart. FIGS. 26A and 26B show magnetic resonanceimaging (MRI) reconstructions of the implant positions within theporcine tissue volumes. FIG. 26A shows an MRI reconstruction of theconfiguration for power transfer across a porcine chest wall to animplant located on the heart surface. A T2-weighted spin-echo pulsesequence was used to acquire the MRI image, and the image wasreconstructed using the OsiriX software package. The source center(white dot) and the coil of the implant (gray dot) were 5 cm apart (1 cmair gap, 4 cm heterogeneous tissue). The fields in FIG. 26A werecalculated using a commercial electromagnetic simulator. The patternedmetal plate was placed above a tissue multilayer (1 cm air gap, 4 mmskin, 8 mm fat, 8 mm muscle, 16 mm bone, 144 mm heart) and the fieldswere calculated by a time-domain solver.

FIG. 26B shows an MRI reconstruction of the configuration for powertransfer to an implant located in the lower cortex region of a porcinebrain. T2-weighted fast spin-echo was used to acquire the MRI image, andthe image was reconstructed using the OsiriX software package. In theconfiguration shown there, the source-implant separation was 5.5 cm.When coupling 500 mW into tissue (approximately the output power of cellphones), the power transferred to the coil of the implant was measuredto be 195 μW for the implant located on the heart surface, and 200 μWfor the implant located in the lower cortex. The received power remainedsubstantial (˜10 μW) even when the operating depth (i.e., the distancebetween the source and the implant) was increased to 10 cm.

These levels are far greater than requirements for advanced integratedcircuits. To illustrate the range of applications available withperformance characteristics reported in the main paper, Table 1 belowdescribes the power requirements of selected state-of-the-art integratedcircuits. The table is not exhaustive, but is representative of existingsolid-state circuit capabilities in the microwatt power regime. Most ofthese devices are currently powered with either wire tethers or large(>2 cm) near-field coils.

TABLE 1 Fabrication process and power consumption of selected integratedelectronics Function Fabrication Process Power Consumption Neural localfield  0.8 μm CMOS 4.5 μW per channel potential sensing Optogeneticstimulation 0.8 μm HV CMOS 400 μW Neural recording 0.18 μm CMOS 0.73 μWper channel Pacemaker  0.5 μm CMOS  8 μW Intracardiac impedance 0.18 μmCMOS 6.67 μW per channel measurement Fluorimeter  0.6 μm CMOS <1 μJ permeasurement Intraocular pressure 0.18 μm CMOS 1.44 μJ per measurementsensor Temperature sensor 0.16 μm CMOS 0.027 μJ per CMOS image sensor0.18 μm CMOS 3.4 μJ per frame Locomotion   65 nm CMOS 250 μW at 0.53cm/s

In comparison, cardiac pacemakers consume about 8 μW. Provided that thefields can be refocused, computational studies show that the performanceis insensitive to the fine structure and composition of the intermediatetissue.

The excess energy dissipated over tissue may pose potential safetyconcerns. The basic metric for radio-frequency exposure is the specificabsorption rate (SAR), defined as the power loss integral over areference volume of tissue. Limits exist on the SAR induced by a sourceof electromagnetic fields in order to protect against adverse healtheffects arising from tissue heating. A system is compliant with the IEEEC95.3-2005 standard if (i) the whole-body average SAR is less than 0.4W/kg and (ii) the maximum local SAR (averaged over 10 g of tissue) doesnot exceed 10 W/kg. These limits are reduced by a factor of 5 forgeneral public exposure (uncontrolled environments), such as for cellphones.

To assess the exposure levels induced by power transfer, a source asdescribed herein was operated over a simulated tissue volume defined byan anthropomorphic fiberglass shell. The spatial distribution ofabsorbed power was measured by scanning a robotic probe throughdosimetric liquids mimicking the body and head, as shown in FIG. 27A.When coupling 500 mW of focused power into tissue, the maximum specificabsorption rate (SAR) was found to be 0.89 W/kg for the body and 1.17W/kg for the head, averaged over 10 g of tissue, as shown in FIGS. 27Band 27C. These levels are far below the exposure threshold forcontrolled environments, as shown in FIG. 27D. If the power coupled intotissue is allowed to meet the maximum permitted level of exposure, FIG.27E shows that 2.2 mW and 1.7 mW can be transferred for theconfigurations shown in FIGS. 26A and 26B, respectively. The lowbody-averaged absorption (<0.04 W/kg for adult humans) and localizeddistribution suggest that the power transfer is unlikely to have ameaningful impact on core body temperatures. FIG. 27F shows that thepower levels are at or below the safety threshold.

Implantable Midfield Receiver

Previous through tissue wireless power transmission techniques, wherethe transmitter and receiver are within a wavelength (in air) of eachother rely on coupling where the dominant field type in the near fieldof the transmitter and receiver structures are the same. For example, anexternal transmitter loop can transmit a magnetic field that isinductively coupled through the magnetic fields to an implanted receiverloop. In another example, an external dipole can be coupled withelectric fields to an implanted dipole.

Using a midfield external transmitter, however, an electric field basedreceiver (e.g., a dipole antenna) can be coupled with a magnetic field(e.g., a tangential H-field) based transmitter. With a strong tangentialH-field component, the magnetic field can propagate through the tissuemedium. With a midfield transmitter, the electric field and magneticfields are proportional in the induced propagating waves. A midfieldtransmitter with a strong magnetic field component can be coupled to anelectric field based receiver.

Previous receiver antennas that were coupled to midfield transmittersinclude a helical structure. In contrast to the helical structure whichrequires a three dimensional production technique, a dipole can beeasily manufactured, such as on a planar surface. Also, the dipole canbe more easily integrated into an injectable (e.g., long and thin)implant than the helical structure.

FIG. 29 shows an implantable apparatus 3000 implanted in tissue 3001.The implantable apparatus 3000 as illustrated includes a dipole antenna3002 and receiver 3004 encapsulated in a material 3006 and an outerimplant casing 3010. The implantable apparatus 3000 also includesoptional surface electrodes 3008 on the outer implant casing 3010. Thesurface electrodes 3008 are optional. For example, when the apparatus3000 is being used as an ablation device, the surface electrodes 3008transfer received energy to the tissue 3001. However, in an applicationin which the receiver is used to help provide power for electronicswithin an implanted device, for example an implanted sensor, theelectrodes 3008 may not be needed.

The dipole antenna 3002 is made of a conductive material, such as ametal, semiconductor, polymer, or other conductive material. The dipoleantenna 3002 can include two straight, thin conductors as shown in FIG.29, or can include other dipole antenna shapes, such as a folded dipole,short dipole, cage dipole, bow-tie dipole, or batwing dipole. Using ashape other than the straight dipole will generally increase the width(e.g., diameter) of the apparatus 3000 relative to the diameter of theapparatus that includes a straight, thin dipole.

The receiver 3004 can be any receiver capable of receiving a signal froma midfield coupler. In one or more embodiments, the receiver can be anultra-high frequency (UHF) receiver, such as is capable of receivingsignals transmitted at a frequency of about 2.45 GHz. The wavelength ofsuch signals in air is about 12.25 centimeters.

The material 3006 can be a high dielectric, low loss material, forexample, PREPERM®, polytetrafluoroethylene (PTFE), such as a highdielectric PTFE, an Eccostock® material, or RT/Duroid®. The material3006 can have a dielectric permittivity between the dielectricpermittivity of the midfield coupler substrate 1312 (see FIG. 13) andthe dielectric permittivity of the tissue 3001 in which the material3006 is implanted. Such a configuration can allow for a larger receiverthan would be allowed by a purely tissue loaded receiver, because one ormore receiver dimensions are generally proportional to a wavelength ofthe signal incident thereon. A purely tissue coupled receiver is thussmall as compared to a receiver that includes a dielectric with apermittivity between the dielectric permittivity of the midfield couplersubstrate 1312 and the dielectric permittivity of the tissue 3001 inwhich the material 3006 is implanted. This in turn can increaseefficiency of the power transmission link.

Consider an implantable receiver coupled to a small antenna encapsulatedin a low dielectric material. The receiver is implanted in tissue thathas a large dielectric permittivity relative to the low dielectricpermittivity encapsulant. A large power loss is realized between thehigh dielectric tissue and the low dielectric encapsulant. To reducethis loss, the material 3106 can have a dielectric permittivity closerto that of the surrounding tissue, such as to better match the perceivedimpedances of the tissue and the encapsulant. In general, when using ahigher dielectric material as an encapsulant, the receiver circuit has asmaller perceived impedance change than when using a lower dielectricmaterial as an encapsulant. In other words, assume the impedance of thereceiver in air is “d1” and the impedance of the receiver in tissue is“d2”. Assume also, that the impedance difference, |d2−d1|=delta1 for thehigh impedance encapsulant and |d2−d1|=delta2 for the low impedanceencapsulant. Generally, delta2>delta1. The smaller impedance changeallows the dynamic range of an adaptive impedance matching network(e.g., a programmable inductor and/or programmable capacitor) at theinterface of the receiver to be reduced. Another advantage of using anencapsulant with a higher dielectric permittivity includes the receiverbeing less sensitive to changes in surrounding tissue dielectricproperties, which may occur due to scar tissue or adipose tissueformation.

The electrodes 3008 are optional and are electrically conductiveelements that are electrically coupled to the receiver 3004. Theelectrodes 3008 transfer energy (electrical field energy) received atthe receiver 3004 to the tissue 3001 in contact with the electrodes3008, such as to ablate the tissue 3001.

The outer implant casing 3010 encloses the dipole antenna 3002, thereceiver 3004, and the encapsulant material 3006. In one or moreembodiments, the outer implant casing 3010 can be made of polyurethane,silicone, ceramics, other urethane blends, Tecothane®, Polyether etherketone (PEEK), Pebax®, nylon, polycarbonate, Acrylonitrile butadienestyrene (ABS), thermoplastic, epoxy, combinations thereof, or the like.

Phase and/or Amplitude Adjustment for Transmitter

Previous solutions to help focus energy on an implanted receiver includea power detector integrated into the implant, as previously described.When using a time domain multiplexing communication system between anexternal transmitter and an implanted receiver, the phase and amplitudecan be dynamically adjusted to help focus energy (e.g., more efficientlyfocus energy) at the implanted receiver without using a power detectorat the implant.

FIG. 30 shows a time domain multiplexed communication system 3100. Thesystem 3100 as illustrated includes an external midfield transceiver3102 and an implantable transceiver 3104. The transceiver 3102 includesa communicatively coupled midfield antenna 3106 and the transceiver 3104includes a communicatively coupled electric field based antenna 3108.The antennas 3106 and 3108 can be configured (e.g., in length, width,shape, material, etc.) to transmit and receive signals at the samefrequency. The transceiver 3104 can transmit data signals through theantenna 3108 to the transceiver 3102 and can receive power and datasignals transmitted by the transceiver 3102 through the antenna 3106.

The external midfield coupler (external transmitter) and implanttransceiver (that includes the implant antenna) can be used for bothtransmission and reception of RF signals. T/R switches can be used toswitch each RF port of the external transmitter from transmit (transmitdata or power) to receive (receive data) mode (see FIG. 31). A T/Rswitch can be used to switch the implant between transmit (datatransmission mode) and receive (power or data receive) mode (see FIG.31).

The output of the receive terminal (on the external transmitter) of theT/R switch can be connected to one or more components that detect thephase and/or amplitude of the received signal from the implant. Thisphase and amplitude information can be used to program the phase of thetransmit signal to be substantially the same relative phase as thereceived signal. To help achieve this, the transceiver 3102 can includea phase and amplitude matching network 3200, such as is shown in FIG.31. The network 3200 is for use with a midfield coupler that includesfour ports, such as the midfield coupler 602 of FIG. 6C. The network3200 as illustrated includes a midfield coupler 3202 electricallycoupled to a plurality of switches 3204A, 3204B, 3204C, and 3204D. Theswitches 3204A-D are each electrically coupled to a phase and/oramplitude detector 3206A, 3206B, 3206C, and 3206D, and a variable gainamplifier 3208A, 3208B, 3208C, and 3208D, respectively. The amplifier3208A-D is electrically coupled to a phase shifter 3210A, 3210B, 3210C,and 3210D, respectively and the phase shifter 3210A-D is electricallycoupled to a power divider 3212 that receives an RF input signal 3214 tobe transmitted through the midfield coupler 3202.

The midfield coupler 3202 can be any midfield coupler discussed herein.The switch 3204A-D can be a selector switch that selects either thereceive line (“R”) or the transmit line (“T”). The number of switches3204A-D of the network 3200 can be equal to the number of ports of themidfield coupler 3202. In the example of the network 3200 the midfieldcoupler 3202 has four ports, however any number of ports (and switches),one or more, can be used. In the example of a midfield coupler with asingle port, the power divider 3212 is superfluous.

The phase and/or amplitude detector 3206A-D detects the phase (φ₁, φ₂,φ₃, φ₄) and power (P₁, P₂, P₃, P₄) of a signal received at each port ofthe midfield coupler 3202. The phase and/or amplitude detector 3206A-Dcan be implemented in one or more modules (hardware modules that caninclude electric or electronic components arranged to perform anoperation, such as determining a phase or amplitude of a signal), suchcan include a phase detector module and/or an amplitude detector module.The detector 3206A-D can include analog and/or digital componentsarranged to determine the phase and/or amplitude of a signal received atthe midfield detector 3202.

The amplifier 3208A-D can receive an input (e.g., M) from the phaseshifter 3210A-D (e.g., P_(k) phase shifted by Φ₁+Φ_(k), Φ₂+Φ_(k),Φ₃+Φ_(k), or Φ₄+Φ_(k)). The output of the amplifier, O, is generally theoutput of the power divider, M when the RF signal 3214 has an amplitudeof 4*M in the example of FIG. 31, multiplied by the gain of theamplifier P_(i)*P_(k). P_(k) can be set dynamically as the values forP₁, P₂, P₃, and/or P₄ change. Φ_(k) is a constant. The phase shifter3210A-D sets the relative phases of the ports based on the phase fromthe detector 3206A-D.

Consider a situation in which the transmit power required to betransmitted from the midfield coupler 3202 is P_(tt). The RF signalprovided to the power divider 3212 has a power of 4*M. The output of theamplifier 3208A is generally M*P₁*P_(k). Thus, the power transmittedfrom the midfield coupler isM*(P₁*P_(k)+P₂*P_(k)+P₃*P_(k)+P₄*P_(k))=P_(tt). Solving for P_(k) yieldsP_(k)=P_(tt)/(M*(P₁+P₂+P₃+P₄)).

The amplitude of a signal at each RF port can be transmitted with thesame relative (scaled) amplitude as the signal received at therespective port of the midfield coupler coupled thereto. The gain of theamplifier 3208A-D can be further refined to account for a loss betweenthe transmission and reception of the signal from the midfield coupler.Consider a reception efficiency of η=P_(ir)/P_(tt), where P_(ir) is thepower received at the implanted receiver. An efficiency (e.g., a maximumefficiency), given a phase and amplitude tuning, can be estimated fromthe amplitude received from the transmitter of the implant at theexternal midfield coupler. This estimation can be given asη≈(P₁+P₂+P₃+P₄)/P_(it), where P_(it) is the original power of the signalfrom the implanted transmitter. The power of the signal from theimplanted transmitter can be communicated to the external transceiver3102 as data from the implanted transceiver 3104. The amplitude of asignal received at an amplifier 3108A-D can be scaled according to thedetermined efficiency to help ensure that the implant receives power toperform the programmed operation(s). Given the estimated linkefficiency, η, and an implant power (e.g., amplitude) requirement ofP_(ir)′, P_(k) can be scaled as P_(k)=P_(ir′)/[η(P₁+P₂+P₃+P₄)] to helpensure that the implant receives adequate power to perform theprogrammed functions.

The control signals for the phase shifter 3210A-D and the amplifier3208A, such as the phase input and gain input, respectively, can beprovided by processing circuitry that is not shown in FIG. 31. Thecircuitry is omitted so as to not overly complicate or obscure the viewprovided in FIG. 31. The same or different processing circuitry can beused to change the switch 3204A-D from the receive line to the transmitline and vice versa. Again, this processing circuitry is not shown inFIG. 31 so as to not overly complicate or obscure the view provided inFIG. 31. See control unit 2112 of FIG. 21A for an example of suchprocessing circuitry.

Customizing Midfield Coupler Dimension(s)

Every body is different in terms of structure (e.g., tissue, muscledensity, fat content, cartilage, scar tissue, tendon makeup, or otherstructure properties, such as bone), contour, and/or shape. Differentmidfield coupler shapes can provide a variety of characteristics thathelp to more efficiently accommodate power transfer to receivers in suchbodies and/or fit comfortably on the external surface (e.g., the skin,such as an epidermal layer) of the body. A midfield coupler with a firstshape may be more efficient at delivering power to a first body, butless efficient at delivering power to a second body.

FIG. 32 shows a midfield coupler attached to tissue (e.g., human orother animal skin). The midfield coupler system 3300 as illustratedincludes a midfield coupler 3302, an electronics module 3304, RFconnectors 3306A and 3306B, and a molded backing layer 3308. The system3300 is shown attached to tissue 3310.

The operations shown in FIG. 33 show a method of designing a midfieldcoupler to accommodate a specific body shape, contour, and/or structure.This procedure allows for design of a focused midfield transmitter forefficiently powering or otherwise providing energy to an anatomicalstructure, such as when a high field intensity is required to power morepower hungry electronics or perform an ablation at a target region, andwhen the implant is implanted deeper than the near field, such as in themid-field.

At operation 3402, an anatomical structure (e.g., the structure 3310)can be imaged, such as by using Magnetic Resonance Imaging (MRI) device,a Computed Tomography (CT) device, or other imaging device. Theanatomical structure includes the area at which the implant is to besituated. At operation 3404, the imaged structure can be decomposed intoa simplified model of geometric shapes of materials (e.g., tissue, bone,tendon, cartilage, scar tissue, organs, fluids, and/or vessels, etc.)with known dielectric properties. At operation 3406, a currentdistribution at a target frequency (e.g., 915 MHz, 2.45 GHz, or othermicrowave frequency) is determined. At operation 3408 dimensions of amidfield coupler (e.g., width/length of strip, slot width/length,spacing between slot(s), value of one or more passive components (e.g.,programmable passive components, such as a capacitor or inductor), suchas can be used for impedance matching, or ports of midfield source, suchas the midfield coupler 3302) that can provide energy at or near thedetermined current distribution are determined. Not all currentdistributions may be possible, so it may be necessary to choose adifferent implantation site, or to operate the midfield coupler at lessthan optimal efficiency. The current distribution can be determined bysolving a current distribution equation previously discussed.

At operation 3410 a customized midfield plate of a midfield coupler canbe created (e.g., etched, plated, and/or printed). The plate can becreated by using a standard fabrication technique and or materials, suchas can include FR4, polyimide, or other material. At operation 3412,electric/electronic components can be electrically coupled to themidfield coupler. The components can include one or more connectors,such as the RF connectors 3306A-B, electric/electronics module 3304(e.g., one or more transistors, resistors, capacitors, transceivers(e.g., transmit and receive radio and antenna), inductors, digitallogic, such as logic gates (e.g., programmable logic gates), anArithmetic Logic Unit (ALU), a processor, or the like). Theelectric/electronic module 3304 can include the switches 3204A-D,detector 3206A-D, the amplifier 3208A-D, the phase shifter 3210A-D,and/or the power divider 3212.

At operation 3414, an insulation material, such as the material 3308,can be attached to the midfield coupler 3302. The insulation materialcan include foam, polymer (e.g., plastic), or silicone. The material3308 can be attached to the surface of the midfield coupler 3302 toprovide an insulation layer between the midfield coupler and the tissue3310. The material 3308 can be molded, cut, or 3D printed to conform tothe shape of the skin surface. Imaging the contour of the surface can bedone using a camera, laser, or cast. Conforming the material to thetissue 3310 can increase the comfort of the patient while wearing thetransmitter and minimizing slipping or displacement of the transmitterfrom the target anatomy. Minimizing slipping can be important fornon-adjustable midfield couplers in which the focal region of thetransmitter is fixed. Additional backing material may be added toprovide a soft interface between skin and the midfield coupler unit.

More Applications

One or more of the systems, apparatuses, and methods discussed hereincan be used to help treat fecal or urinary incontinence (e.g.,overactive bladder), such as by stimulating the tibial nerve or anybranch of the tibial nerve, such as but not limited to the posteriortibial nerve, one or more nerves or nerve branches originating from thesacral plexus, including but not limited to S1-S4, the tibial nerve,and/or the pudendal nerve. One or more of the systems, apparatuses, andmethods discussed herein can be used to help treat urinary incontinence,such as overactive bladder. Urinary incontinence can be treated by usingmidfield wireless transfer by stimulating one or more of muscles of thepelvic floor, nerves innervating the muscles of the pelvic floor,internal urethral sphincter, external urethral sphincter, and thepudendal nerve or branches of the pudendal nerve.

One or more of the systems, apparatuses, and methods discussed hereincan be used to help treat sleep apnea and/or snoring by stimulating oneor more of a nerve or nerve branches of the hypoglossal nerve, the baseof the tongue (muscle), phrenic nerve(s), intercostal nerve(s),accessory nerve(s), and cervical nerves C3-C6. Treating sleep apneaand/or snoring can include using a midfield coupler to provide energy toan implant to sense a decrease, impairment, or cessation of breathing(such as by measuring oxygen saturation).

One or more of the systems, apparatuses, and methods discussed hereincan be used to help treat vaginal dryness, such as by stimulating one ormore of bartholin gland(s), skene's gland(s), and inner wall of vagina.One or more of the systems, apparatuses, and methods discussed hereincan be used to help treat a migraine, such as by stimulating one or moreof the occipital nerve, supraorbital nerve, C2 cervical nerve, orbranches thereof, and the frontal nerve, or branches thereof. One ormore of the systems, apparatuses, and methods discussed herein can beused to help treat post-traumatic stress disorder, hot flashes, and/orcomplex regional pain syndrome such as by stimulating one or more of thestellate ganglion and the C4-C7 of the sympathetic chain.

One or more of the systems, apparatuses, and methods discussed hereincan be used to help treat trigeminal neuralgia, such as by stimulatingone or more of the sphenopalatine ganglion nerve block, the trigeminalnerve, or branches of the trigeminal nerve. One or more of the systems,apparatuses, and methods discussed herein can be used to help treat drymouth (e.g., caused by side effects from medications, chemotherapy orradiation therapy cancer treatments, Sjogren's disease, or by othercause of dry mouth), such as by stimulating one or more of Parotidglands, submandibular glands, sublingual glands, submucosa of the oralmucosa in the oral cavity within the tissue of the buccal, labial,and/or lingual mucosa, the soft palate, the lateral parts of the hardpalate, and/or the floor of the mouth and/or between muscle fibers ofthe tongue, Von Ebner glands, glossopharyngeal nerve (CN IX), includingbranches of CN IX, including otic ganglion, a facial nerve (CN VII),including branches of CN VII, such as the submandibular ganglion, andbranches of T1-T3, such as the superior cervical ganglion.

One or more of the systems, apparatuses, and methods discussed hereincan be used to help treat a transected nerve, such as by sensingelectrical output from the proximal portion of a transected nerve anddelivering electrical input into the distal portion of a transectednerve, and/or sensing electrical output from the distal portion of atransected nerve and delivering electrical input into the proximalportion of a transected nerve. One or more of the systems, apparatuses,and methods discussed herein can be used to help treat cerebral palsy,such as by stimulating one or more muscles or one or more nervesinnervation one or more muscles affected in a patient with cerebralpalsy. One or more of the systems, apparatuses, and methods discussedherein can be used to help treat erectile dysfunction, such as bystimulating one or more of pelvic splanchnic nerves (S2-S4) or anybranches thereof, the pudendal nerve, cavernous nerve(s), and inferiorhypogastric plexus.

One or more of the systems, apparatuses, and methods discussed hereincan be used to help treat menstrual pain, such as by stimulating one ormore of the uterus and the vagina. One or more of the systems,apparatuses, and methods discussed herein can be used as an intrauterinedevice, such as by sensing one or more PH and blood flow or deliveringcurrent or drugs to aid in contraception, fertility, bleeding, or pain.One or more of the systems, apparatuses, and methods discussed hereincan be used to incite human arousal, such as by stimulating femalegenitalia, including external and internal, including clitoris or othersensory active parts of the female, or by stimulating male genitalia.One or more of the systems, apparatuses, and methods discussed hereincan be used to help treat hypertension, such as by stimulating one ormore of a carotid sinus, vagus nerve, or a branch of the vagus nerve.One or more of the systems, apparatuses, and methods discussed hereincan be used to help treat paroxysmal supraventricular tachycardia, suchas by stimulating one or more of trigeminal nerve or branches thereof,anterior ethmoidal nerve, and the vagus nerve.

One or more of the systems, apparatuses, and methods discussed hereincan be used to help treat vocal cord dysfunction, such as by sensing theactivity of a vocal cord and the opposite vocal cord or just stimulatingone or more of the vocal cords by stimulating nerves innervating thevocal cord, the left and/or Right recurrent laryngeal nerve, and thevagus nerve. One or more of the systems, apparatuses, and methodsdiscussed herein can be used to help repair tissue, such as bystimulating tissue to do one or more of enhancing microcirculation andprotein synthesis to heal wounds and restoring integrity of connectiveand/or dermal tissues. One or more of the systems, apparatuses, andmethods discussed herein can be used to help asthma or chronicobstructive pulmonary disease, such as by one or more of stimulating thevagus nerve or a branch thereof, blocking the release of norepinephrineand/or acetylcholine and/or interfering with receptors fornorepinephrine and/or acetylcholine. One or more of the systems,apparatuses, and methods discussed herein can be used to help treatcancer, such as by stimulating, to modulate one or more nerves near orin a tumor, such as to decrease the sympathetic innervation, such asepinephrine/NE release, and/or parasympathetic innervation, such as Ach.One or more of the systems, apparatuses, and methods discussed hereincan be used to help treat diabetes, such as by powering a sensor insidethe human body that detects parameters of diabetes, such as a glucoselevel or ketone level and using such sensor data to adjust delivery ofexogenous insulin from an insulin pump. One or more of the systems,apparatuses, and methods discussed herein can be used to help treatdiabetes, such as by powering a sensor inside the human body thatdetects parameters of diabetes, such as a glucose level or ketone level,and using a midfield coupler to stimulate the release of insulin fromislet beta cells.

Additional Examples

Example 1 can include or use subject matter (such as an apparatus, amethod, a means for performing operations, or a machine readable memoryincluding instructions that, when performed by the machine, canconfigure the machine to perform acts), such as can include or use afirst transceiver that transmits and receives microwave signals at afirst frequency, the first transceiver including a midfield coupler thatconverts signals from the first transceiver to signals with anon-negligible H-field component parallel to a surface of the midfieldcoupler and focuses the converted signals to a location within tissuethat is within a wavelength, as measured in air, of the microwavesignals; and an at least partially implantable biocompatible devicecomprising a second transceiver, the second transceiver including anE-field based antenna that receives the signals from the midfieldcoupler and the second transceiver transmits signals at about the samefrequency as the first transceiver.

Example 2 can include or use, or can optionally be combined with thesubject matter of Example 1, to include or use, wherein the E-fieldbased antenna is a dipole antenna.

Example 3 can include or use, or can optionally be combined with thesubject matter of at least one of Examples 1-2, to include or use,wherein the first transceiver comprises a phase matching networkcomprising a phase detector and a phase shifter, the phase detector andthe phase shifter electrically coupled to the midfield coupler, thephase detector determines a phase of a signal received from the secondtransceiver, and the phase shifter adjusts a phase of a signal to beprovided to the midfield coupler based on the determined phase of thesignal received from the second transceiver.

Example 4 can include or use, or can optionally be combined with thesubject matter of Example 3, to include or use, wherein the phaseshifter adjusts the phase of the signal by the determined phase of thesignal received from the second transceiver.

Example 5 can include or use, or can optionally be combined with thesubject matter of Example 3, to include or use, wherein the phaseshifter adjusts the phase of the signal to match the phase of the signalreceived from the second transceiver.

Example 6 can include or use, or can optionally be combined with thesubject matter of at least one of Examples 1-5, to include or use,wherein the first transceiver comprises an amplitude matching networkcomprising an amplitude detector and a variable gain amplifierelectrically coupled to the midfield coupler, the amplitude detectordetermines an amplitude of a signal received from the second transceiverand the variable gain amplifier adjusts an amplitude of a signal to beprovided to the midfield coupler based on the amplitude of the signalreceived from the second transceiver.

Example 7 can include or use, or can optionally be combined with thesubject matter of Example 6, to include or use, wherein the midfieldcoupler includes two or more ports, the amplitude detector is one of twoor more amplitude detectors, each amplitude detector of the two or moreamplitude detectors electrically coupled to a respective port of themidfield coupler, the first transceiver further comprises a powerdivider which receives a radio frequency (RF) signal and divides andseparates the RF signal into two or more signals, one signal for eachport of the midfield coupler, and wherein the variable gain amplifier isone of a plurality of variable gain amplifiers, each variable gainamplifier is electrically coupled between a respective port of themidfield coupler and the power divider, each amplifier receives a signalof the two or more signals from the power divider and amplifies thesignal by a gain, wherein the gain is determined based on an amplitudedetermined by the amplitude detector coupled to the same respective ofthe midfield coupler.

Example 8 can include or use, or can optionally be combined with thesubject matter of Example 7, to include or use, wherein the gain of eachamplifier of the plurality of amplifiers is the amplitude determined bythe amplitude detector multiplied by a quantity.

Example 9 can include or use, or can optionally be combined with thesubject matter of Example 8, to include or use, wherein the quantity isP_(k)=P_(tt)/Σ_(i=1) ^(N)P_(i), where P_(tt) is a specified amplitudeand P_(i) is an amplitude of the plurality of amplitudes determined atthe amplitude detector for each of the i ports of the midfield coupler.

Example 10 can include or use, or can optionally be combined with thesubject matter of Example 9, to include or use, wherein the quantity,Pk, is further divided by an efficiency indicator, η, where η=Σ_(i=1)^(N)P_(i)/P_(it) where P_(it) is an amplitude of a signal transmittedfrom the second transceiver.

Example 11 can include or use, or can optionally be combined with thesubject matter of at least one of Examples 1-10, to include or use,wherein the antenna is encapsulated in a dielectric material with adielectric permittivity between a dielectric permittivity of animaltissue and a dielectric permittivity of a substrate of the midfieldcoupler on which a midfield plate of the midfield coupler is arranged.

Example 12 can include or use subject matter (such as an apparatus, amethod, a means for performing operations, or a machine readable memoryincluding instructions that, when performed by the machine, canconfigure the machine to perform acts), such as can include or use aradio that transmits and receives microwave signals, a midfield couplerelectrically coupled to the radio, the midfield coupler converts signalsfrom the radio to signals with a non-negligible H-field componentparallel to a surface of the midfield coupler and focuses the signals toa location within tissue within a wavelength of the microwave signals asmeasured in air, an amplitude detector electrically coupled to themidfield coupler, the amplitude detector determines an amplitude of asignal received at the midfield coupler, and a variable gain amplifierelectrically coupled between the radio and the midfield coupler, theamplifier to amplify a transmit signal from the radio in proportion tothe amplitude determined by the amplitude detector.

Example 13 can include or use, or can optionally be combined with thesubject matter of Example 12, to include or use a phase matching networkcomprising a phase detector and a phase shifter, the phase shifter andthe phase detector electrically coupled to the midfield coupler, thephase detector determines a phase of a signal received at the midfieldcoupler and the phase shifter adjusts a phase of a signal provided tothe midfield coupler based on the determined phase.

Example 14 can include or use, or can optionally be combined with thesubject matter of Example 13, to include or use, wherein the phaseshifter adjusts the phase of the signal by the determined phase.

Example 15 can include or use, or can optionally be combined with thesubject matter of at least one of Examples 12-14, to include or use,wherein the midfield coupler includes two or more ports, the amplitudedetector is one of two or more amplitude detectors, each amplitudedetector of the two or more amplitude detectors electrically coupled toa respective port of the midfield coupler, the first transceiver furthercomprises a power divider which receives a radio frequency (RF) signaland divides and separates the RF signal into two or more signals, onesignal for each port of the midfield coupler, and wherein the variablegain amplifier is one of a plurality of variable gain amplifiers, eachvariable gain amplifier is electrically coupled between a respectiveport of the midfield coupler and the power divider, each amplifierreceives a signal of the two or more signals from the power divider andamplifies the signal by a gain, wherein the gain is determined based onan amplitude determined by the amplitude detector.

Example 16 can include or use, or can optionally be combined with thesubject matter of Example 15, to include or use, wherein the gain ofeach amplifier of the plurality of amplifiers is the amplitudedetermined by the amplitude detector multiplied by a quantity.

Example 17 can include or use, or can optionally be combined with thesubject matter of Example 16, to include or use, wherein the quantity isP_(k)=P_(tt)/(η*(Σ_(i=1) ^(N)P_(i))), where P_(tt) is a specifiedamplitude, P_(i) is an amplitude of the plurality of amplitudesdetermined at the amplitude detector for each of the i ports of themidfield coupler and η=Σ_(i=1) ^(N)P_(i)/P_(it) where P_(it) is anamplitude of a signal transmitted to the midfield coupler.

Example 18 can include or use subject matter (such as an apparatus, amethod, a means for performing operations, or a machine readable memoryincluding instructions that, when performed by the machine, canconfigure the machine to perform acts), such as can include or use an atleast partially implantable, biocompatible apparatus comprising, anouter casing, a radio that transmits and receives microwave signalsencased by the outer casing, an electric field based antennaelectrically coupled to the radio and encased by the outer casing, andan encapsulant within the outer casing, the encapsulant surrounding theradio and the antenna and the encapsulant including a dielectricpermittivity between a dielectric permittivity of animal tissue and adielectric permittivity of a substrate of a midfield coupler.

Example 19 can include or use, or can optionally be combined with thesubject matter of Example 18, to include or use, wherein the antenna isa dipole antenna.

Example 20 can include or use, or can optionally be combined with thesubject matter of at least one of Examples 18-19, to include or use, oneor more electrodes exposed on the outer casing and electrically coupledto the radio.

What is claimed is:
 1. A system comprising: an implantable stimulationdevice; a midfield powering system wirelessly coupled to the implantablestimulation device, the midfield powering system including: a radioconfigured to transmit and receive microwave signals; a midfield couplerelectrically coupled to the radio, the midfield coupler configured toconvert signals from the radio to signals with a non-negligible H-fieldcomponent parallel to a surface of the midfield coupler and focus thesignals to a location in tissue associated with the implantablestimulation device that is within a wavelength of the microwave signalsas measured in air; an amplitude detector electrically coupled to themidfield coupler, the amplitude detector configured to determine anamplitude of a signal transmitted by the implantable stimulation deviceand received at the midfield coupler; a variable gain amplifierelectrically coupled between the radio and the midfield coupler, thevariable gain amplifier configured to amplify a transmit signal from theradio in proportion to the amplitude determined by the amplitudedetector, the transmit signal received by the implantable stimulationdevice; and a phase matching network comprising a phase detector and aphase shifter, the phase shifter and the phase detector electricallycoupled to the midfield coupler, the phase detector configured todetermine a phase of a signal transmitted by the implantable stimulationdevice and received at the midfield coupler and the phase shifterconfigured to adjust a phase of a signal provided to the midfieldcoupler based on the determined phase.
 2. The system of claim 1, whereinthe phase shifter is configured to adjust the phase of the signal by thedetermined phase.
 3. The system of claim 1, wherein: the midfieldcoupler includes two or more ports, the amplitude detector is one of twoor more amplitude detectors, each amplitude detector of the two or moreamplitude detectors electrically coupled to a respective port of themidfield coupler, the system further comprises a power dividerconfigured to receive a radio frequency (RF) signal and divide andseparate the RF signal into two or more signals, one signal for eachport of the midfield coupler, and wherein the variable gain amplifier isone of a plurality of variable gain amplifiers, each variable gainamplifier is electrically coupled between a respective port of themidfield coupler and the power divider, each variable gain amplifierconfigured to receive a signal of the two or more signals from the powerdivider and amplify the signal by a gain, wherein the gain is determinedbased on an amplitude determined by the amplitude detector.
 4. Thesystem of claim 3, wherein the gain of each variable gain amplifier ofthe plurality of variable gain amplifiers is the amplitude determined bythe amplitude detector multiplied by a quantity.
 5. The system of claim4, wherein the quantity is P_(k)=P_(tt)/(n*(Σ_(i=1) ^(N)P_(i))), whereP_(tt) is a specified amplitude, P_(i) is an amplitude of the pluralityof amplitudes determined at the amplitude detector for each of the iports of the midfield coupler and η=Σ_(i=1) ^(N)P_(i)/P_(it) whereP_(it) is an amplitude of a signal transmitted to the midfield coupler.6. A system comprising: an implantable stimulation device; a radioconfigured to transmit and receive microwave signals to the implantablestimulation device and receive microwave signals from the implantablestimulation device; a midfield coupler electrically coupled to theradio, the midfield coupler configured to convert signals from the radioto signals with a non-negligible H-field component parallel to a surfaceof the midfield coupler and focus the signals to a location in tissuethat is within a wavelength of the microwave signals as measured in air;an amplitude detector electrically coupled to the midfield coupler, theamplitude detector configured to determine an amplitude of a signalreceived at the midfield coupler; and a variable gain amplifierelectrically coupled between the radio and the midfield coupler, thevariable gain amplifier configured to amplify a transmit signal from theradio in proportion to the amplitude determined by the amplitudedetector.
 7. The system of claim 6, further comprising: a phase matchingnetwork comprising a phase detector and a phase shifter, the phaseshifter and the phase detector electrically coupled to the midfieldcoupler, the phase detector configured to determine a phase of a signalreceived at the midfield coupler and the phase shifter configured toadjust a phase of a signal provided to the midfield coupler based on thedetermined phase.
 8. The system of claim 7, wherein the phase shifter isconfigured to adjust the phase of the signal by the determined phase. 9.The system of claim 6, wherein: the midfield coupler includes two ormore ports, the amplitude detector is one of two or more amplitudedetectors, each amplitude detector of the two or more amplitudedetectors electrically coupled to a respective port of the midfieldcoupler, the system further comprises a power divider configured toreceive a radio frequency (RF) signal and divide and separate the RFsignal into two or more signals, one signal for each port of themidfield coupler, and wherein the variable gain amplifier is one of aplurality of variable gain amplifiers, each variable gain amplifier iselectrically coupled between a respective port of the midfield couplerand the power divider, each variable gain amplifier configured toreceive a signal of the two or more signals from the power divider andamplify the signal by a gain, wherein the gain is determined based on anamplitude determined by the amplitude detector.
 10. The system of claim9, wherein the gain of each variable gain amplifier of the plurality ofvariable gain amplifiers is the amplitude determined by the amplitudedetector multiplied by a quantity.
 11. The system of claim 10, whereinthe quantity is P_(k)=P_(tt)*(η*(Σ_(i=1) ^(N)P_(i))), where P_(tt) is aspecified amplitude, P_(i) is an amplitude of the plurality ofamplitudes determined at the amplitude detector for each of the i portsof the midfield coupler and η=Σ_(i=1) ^(N)P_(i)/P_(it) where P_(it) isan amplitude of a signal transmitted to the midfield coupler.
 12. Thesystem of claim 11, further comprising a plurality of switches, each ofthe plurality of switches respectively electrically coupled between arespective amplitude detector of the two or more amplitude detectors anda respective port of the two or more ports.
 13. The system of claim 12,wherein each of the plurality of switches includes a receive pathelectrically coupled between the respective amplitude detector of thetwo or more amplitude detectors and the respective port of the two ormore ports and a transmit path electrically coupled between a respectivevariable gain amplifier of the plurality of variable gain amplifiers andthe respective port of the two or more ports.
 14. A system comprising:an implantable stimulation device; a radio configured to transmit andreceive microwave signals to the implantable stimulation device andreceive microwave signals from the implantable stimulation device; amidfield coupler electrically coupled to the radio, the midfield couplerconfigured to convert signals from the radio to signals with anon-negligible H-field component parallel to a surface of the midfieldcoupler and focus the signals to a location in tissue that is within awavelength of the microwave signals as measured in air; and a phasematching network comprising a phase detector and a phase shifter, thephase shifter and the phase detector electrically coupled to themidfield coupler, the phase detector configured to determine a phase ofa signal received at the midfield coupler and the phase shifterconfigured to adjust a phase of a signal provided to the midfieldcoupler based on the determined phase.
 15. The system of claim 8,wherein the phase shifter is configured to adjust the phase of thesignal by the determined phase.
 16. The system of claim 8 furthercomprising: an amplitude detector electrically coupled to the midfieldcoupler, the amplitude detector configured to determine an amplitude ofa signal received at the midfield coupler; and a variable gain amplifierelectrically coupled between the radio and the midfield coupler, thevariable gain amplifier configured to amplify a transmit signal from theradio in proportion to the amplitude determined by the amplitudedetector.
 17. The system of claim 8, wherein: the midfield couplerincludes two or more ports, the amplitude detector is one of two or moreamplitude detectors, each amplitude detector of the two or moreamplitude detectors electrically coupled to a respective port of themidfield coupler, the system further comprises a power dividerconfigured to receive a radio frequency (RF) signal and divide andseparate the RF signal into two or more signals, one signal for eachport of the midfield coupler, and wherein the variable gain amplifier isone of a plurality of variable gain amplifiers, each variable gainamplifier is electrically coupled between a respective port of themidfield coupler and the power divider, each variable gain amplifierconfigured to receive a signal of the two or more signals from the powerdivider and amplify the signal by a gain, wherein the gain is determinedbased on an amplitude determined by the amplitude detector.
 18. Thesystem of claim 11, wherein the gain of each variable gain amplifier ofthe plurality of variable gain amplifiers is the amplitude determined bythe amplitude detector multiplied by a quantity.
 19. The system of claim12, wherein the quantity is P_(k)=P_(tt)/(η*(Σ_(i=1) ^(N)P_(i))), whereP_(tt) is a specified amplitude, P_(i) is an amplitude of the pluralityof amplitudes determined at the amplitude detector for each of the iports of the midfield coupler and η=Σ_(i=1) ^(N)P_(i)/P_(it) whereP_(it) is an amplitude of a signal transmitted to the midfield coupler.20. The system of claim 13, further comprising a plurality of switches,each of the plurality of switches respectively electrically coupledbetween a respective amplitude detector of the two or more amplitudedetectors and a respective port of the two or more ports.